Align
If there are several equations that align
would be applicable.
\begin{align}
y &= 2x + 3x + 4z + 5z \\
&= 5x + 4z + 5z \\
&= 5x + 9z
\end{align}
Matrix
\left[
\begin{matrix}
x_{11} & x_{12} \\
x_{21} & x_{22}
\end{matrix}
\right]
\left[
\begin{matrix}
x_{11} & \cdots & x_{1n} \\
\vdots & \ddots & \vdots \\
x_{m1} & \cdots & x_{mn}
\end{matrix}
\right]