Let \(C = \begin{bmatrix} 1 & 2 & 3\\4 & 5 & 6\end{bmatrix}
\begin{bmatrix} 1 & -1 \\ 0 & 1\\-1 & 1\end{bmatrix}\).
The answer is 7.
\(C_{2,2}\) is given by the product of the second row of the first
matrix and the second column of the second matrix:
\(\begin{bmatrix} 4 & 5 & 6\end{bmatrix}
\begin{bmatrix} -1 \\ 1\\ 1\end{bmatrix}
= 4 \cdot (-1) + 5 \cdot 1 + 6 \cdot 1=-4 + 5 + 6 = 7\).