Adding and Subtracting Matrices

$A + B = B + A$

$\begin{pmatrix} 2&1\\5&7 \end{pmatrix}+ \begin{pmatrix} 5&3 \end{pmatrix} = \begin{pmatrix} 7&4\\5&7 \end{pmatrix}$

$A+(B+C) = (A+B)+C$

$A + C = B + C$ if and only if $A = B$.

$A + B = B + A$ if and only if $A$ and $B$ are both square matrices.

$A + B$ is a $2m \times 2n$ matrix.