Answer
a) $A+B=\begin{matrix} 0&3&9
\\1&4&-1
\\2&-5&-3\end{matrix}$
b) $A+B=\begin{matrix} -4&9&2&10
\\-4&-5&4&0\end{matrix}$
Work Step by Step
We can add two matrices by summing the (n, m)th element in one with the (n, m)th element in the other.
a) $A+B=\begin{matrix} 1&0&4
\\-1&2&2
\\ 0&-2&-3\end{matrix}+\begin{matrix} -1&3&5
\\2&2&-3
\\2&-3&0\end{matrix}=\begin{matrix} 0&3&9
\\1&4&-1
\\2&-5&-3\end{matrix}$
b)
$A+B=\begin{matrix} -1&0&5&6
\\-4&-3&5&-2\end{matrix}+\begin{matrix}-3&9&-3&4
\\0&-2&-1&2\end{matrix}=\begin{matrix} -4&9&2&10
\\-4&-5&4&0\end{matrix}$