1. ## Determinant of matrix

Is this a 3 by 4 matrix? I know how to find the determinant of a 3 by 3 matrix. How is the determinant calculated for this matrix?

2. You would be trying to evaluate $\displaystyle \left|\begin{matrix}1 & 2 & \phantom{a}-1\\ a & 3 & \phantom{a -}1\\ 2 & 1 & a - 1\end{matrix}\right|$