Look at this matrix. Is it invertible? If so, try compute the inverse.

$$A = \begin{bmatrix} 4 & 7 & 4 \\ 1 & 1 & 2 \\ 5 & 7 & 7 \end{bmatrix}$$
$$A^{-1}=\begin{bmatrix} -7 & -21 & 10 \\ 3 & 8 & -4 \\ 2 & 7 & -3 \end{bmatrix}$$