Equation with cases

Question

How to typeset this equation?

Please help me.

Answer 1

Here you go:

\documentclass{memoir}
\usepackage{amsmath}

\begin{document}

\begin{align*}
    \text{simult\'aneas:}
    \quad
    \begin{cases}
    2x+y=3\\
    2x-y=1\\
    \end{cases}
    \quad
    \Leftrightarrow
    \quad
    x=1,~y=1
\end{align*}


\begin{align*}
    \text{simult\'aneas:}
    \quad
    \begin{cases}
    2x+y=3\\
    2x-y=1\\
    \end{cases}
    \quad
    \Leftrightarrow
    \quad
    x=y=1
\end{align*}

\begin{align*}
    \text{simult\'aneas:}
    \quad
    \begin{cases}
    2x+y=3\\
    2x-y=1\\
    \end{cases}
    \quad
    \Leftrightarrow
    \quad
    \begin{cases}
    x=1\\
    y=1\\
\end{cases}\end{align*}

\end{document}

You might want to change \text to \textsf.

This might be considered a beginner's question, so you might want to have a look on What are good learning resources for a LaTeX beginner?.