Horizontal alignment in enumerate environment

Question

I wanted to align two enumerated item lists horizontally like in the post shown here, however, my alignment is skewed from one item to the other. Bascially, I wanted help to get both items aligned at the "=" sign. Can you help me fix this issue?

Here is my code:

\documentclass{book}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage{mathptmx}
\usepackage{enumitem} % Customize lists

\begin{document}

\begin{enumerate}[align=left,style=nextline,leftmargin=*,labelsep=\parindent,label=\textbf{(\alph*)}]
\item\hfill$\begin{aligned}[t]
P(1) & = 3.1479 \\
K(1) & = 0.786986\\
\hat{x}(1) & = 0 + 0.786986(1.5-0)\\
& = 1.1805
\end{aligned}$\hfill\null\\
\item \hfill$\begin{aligned}[t]
P(2) & = 3.1431\\
K(2) & = 0.7870\\
\hat{x}(2) & = 1.1805 + 0.786986(3.0-1.1805)\\
& = 2.6124
\end{aligned}$\hfill\null
\end{enumerate}

\end{document}

Answer 1

Not very nice input, but not worse than your attempt:

\documentclass{book}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage{mathptmx}
\usepackage{enumitem} % Customize lists

\newenvironment{enumeratealign}
 {\setcounter{enumi}{0}\renewcommand{\theenumi}{\alph{enumi}}\csname flalign*\endcsname}
 {\endalign}
\makeatletter
\newcommand{\alignitem}{%
  \refstepcounter{enumi}%
  \textbf{(\theenumi)}%
  \let\label\ltx@label
}
\makeatletter

\begin{document}

\begin{enumeratealign}
&\alignitem\label{first} &
    P(1)       &= 3.1479 &&\\
&&  K(1)       &= 0.786986 &&\\
&&  \hat{x}(1) &= 0 + 0.786986(1.5-0) &&\\
&&             &= 1.1805 &&\\
\\
&\alignitem &
    P(2)       &= 3.1431 &&\\
&&  K(2)       &= 0.7870 &&\\
&&  \hat{x}(2) &= 1.1805 + 0.786986(3.0-1.1805) &&\\
&&             &= 2.6124 &&
\end{enumeratealign}

\end{document}

Answer 2

This with list is not possible (at least simple) to achieve. I sugest to use flalign math environment:

\documentclass{book}
\usepackage{amsmath,amsfonts,amssymb,amsthm}
\usepackage{mathptmx}

    \begin{document}
\begin{flalign*}
\textbf{(a)}    &&  P(1)        & = 3.1479              &&  \\
                &&  K(1)        & = 0.786986            &&  \\
                &&  \hat{x}(1)  & = 0 + 0.786986(1.5-0) &&  \\
                &&              & = 1.1805              &&  \\[1ex]
\textbf{(b)}    &&  P(2)        & = 3.1431              &&  \\
                &&  K(2)        & = 0.7870              &&  \\
                &&  \hat{x}(2)  & = 1.1805 + 0.786986(3.0-1.1805)
                                                        &&  \\
                &&              & = 2.6124              &&
\end{flalign*}
    \end{document}