Matrices. Know how to add, subtract and multiply matrices. For example, if
$$A = \begin{bmatrix}1 & 2 \\ 3 & 4\end{bmatrix} \quad {\rm {and}} \quad B = \begin{bmatrix}2 & -1 \\ 1 & 2 \end{bmatrix},$$ then
$$AB - B^2 = \begin{bmatrix}a & b \\ c & d \end{bmatrix}$$ where
$a =$ ,
$b =$ ,
$c =$ ,
$d =$ ,