Compilation issue: cannot find the missing $ on line 75, 76

Question

Compiler is yelling at me for a dollar sign I can't find. Do you guys see it at all or have any other options for how to express in-line math equations in problem statements? Thanks in advance.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\documentclass[12pt]{article}
 \usepackage[margin=0.75in]{geometry} 
\usepackage{amsmath,amsthm,amssymb,amsfonts, enumitem, fancyhdr, color, comment, graphicx, environ}
\pagestyle{fancy}
\setlength{\headheight}{65pt}
\newenvironment{problem}[2][Problem]{\begin{trivlist}
\item[\hskip \labelsep {\bfseries #1}\hskip \labelsep {\bfseries #2.}]}{\end{trivlist}}
\newenvironment{sol}
    {\emph{Solution:}
    }
    {
    \qed
    }
\specialcomment{com}{ \color{blue} \textbf{Comment:} }{\color{black}} %for instructor comments while grading
\NewEnviron{probscore}{\marginpar{ \color{blue} \tiny Problem Score: \BODY \color{black} }}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%





%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Fill in the appropriate information below
\lhead{Student}  
\rhead{Professor \\ MAT 2250  \\ 9.18.19 \\ HW 1 Solutions} 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Do not alter this block.
\begin{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



%Copy the following block of text for each problem in the assignment.
\begin{problem}{1.2.11} 
Find the general solution of the system whose augmented matrix is given below.

$$
\quad
\begin{bmatrix}
3 & -4 & 2 & 0 \\
-9 & 12 & -6 & 0 \\
-6 & 8 & -4 & 0 \\
\end{bmatrix}
\quad
$$

\end{problem}


\begin{sol}


\end{sol}


\begin{problem}{1.3.25}
Let $$A = \begin{bmatrix}
1 & 0 & -4 \\
0 & 3 & -2 \\
-2 & 6 & 3 \\
\end{bmatrix}$$ and \textbf{b}=
 $$\begin{bmatrix}

4 \\
1 \\
-4\\

\end{bmatrix}.$$

Denote the columns of $A$ by \textbf{a_1},\textbf{a_2},\textbf{a_3}, and let $W$ = \textrm{Span}\{\textbf{a_1},\textbf{a_2},\textbf{a_3}\}.
\end{problem}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Do not alter anything below this line.
\end{document}

Answer 1

I was curious to see what this looked like so I followed @peter-grill's advice, and the updated code runs fine:

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

\documentclass[12pt]{article}
 \usepackage[margin=0.75in]{geometry} 
\usepackage{amsmath,amsthm,amssymb,amsfonts, enumitem, fancyhdr, color, comment, graphicx, environ}
\pagestyle{fancy}
\setlength{\headheight}{65pt}
\newenvironment{problem}[2][Problem]{\begin{trivlist}
\item[\hskip \labelsep {\bfseries #1}\hskip \labelsep {\bfseries #2.}]}{\end{trivlist}}
\newenvironment{sol}
    {\emph{Solution:}
    }
    {
    \qed
    }
\specialcomment{com}{ \color{blue} \textbf{Comment:} }{\color{black}} %for instructor comments while grading
\NewEnviron{probscore}{\marginpar{ \color{blue} \tiny Problem Score: \BODY \color{black} }}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%





%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Fill in the appropriate information below
\lhead{Student}  
\rhead{Professor \\ MAT 2250  \\ 9.18.19 \\ HW 1 Solutions} 
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Do not alter this block.
\begin{document}
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%



%Copy the following block of text for each problem in the assignment.
\begin{problem}{1.2.11} 
Find the general solution of the system whose augmented matrix is given below.

\[
\quad
\begin{bmatrix}
3 & -4 & 2 & 0 \\
-9 & 12 & -6 & 0 \\
-6 & 8 & -4 & 0 \\
\end{bmatrix}
\quad
\]

\end{problem}


\begin{sol}


\end{sol}


\begin{problem}{1.3.25}
Let \[A = \begin{bmatrix}
1 & 0 & -4 \\
0 & 3 & -2 \\
-2 & 6 & 3 \\
\end{bmatrix}\] and \textbf{b}=
 \[\begin{bmatrix}

4 \\
1 \\
-4\\

\end{bmatrix}.\]

Denote the columns of $A$ by \textbf{$a_1$},\textbf{$a_2$},\textbf{$a_3$}, and let $W$ = \textrm{Span}\{\textbf{$a_1$},\textbf{$a_2$},\textbf{$a_3$}\}.
\end{problem}


%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
%Do not alter anything below this line.
\end{document}