help

Let $\mathbf{A} = \begin{pmatrix} 0 & 5 & 7 \\ -2 & 7 & 7 \\ -1 & 1 & 4 \end{pmatrix}$ Find all values of $l$ such that $\det (t \mathbf{I} - \mathbf{A}) = 0$, where $\mathbf{I}$ is the $3 \times 3$ identity matrix.