(b) A + B + C = \(\begin{bmatrix}x&y\\[0.3em]z&w\end{bmatrix}\)+ \(\begin{bmatrix}x&-y\\[0.3em]-z&w\end{bmatrix}\)+ \(\begin{bmatrix}-2x&0\\[0.3em]0&-2w\end{bmatrix}\)
= \(\begin{bmatrix}x+x+(-2y)&y+(-y)+0\\[0.3em]z+(-z)+0&w+w+(-2w)\end{bmatrix}\)
= \(\begin{bmatrix}0&0\\[0.3em]0&0\end{bmatrix}\)