We summarise the matrix operations for the number $x = 2$ and the square matrices $$A = \begin{pmatrix} 0&1\\ 1&2 \end{pmatrix},\quad\; B = \begin{pmatrix} 1&2\\ 0&-1 \end{pmatrix}$$
Operation Notation Computation Scalar multiplication Addition Multiplication $$x A$$ $$A+B$$ $$A\times B$$ $$\begin{pmatrix} 0&2\\ 2&4 \end{pmatrix}$$ $$\begin{pmatrix} 1&3\\ 1&1 \end{pmatrix}$$ $$\begin{pmatrix} 0&-1\\ 1&0 \end{pmatrix}$$ None Matrices must be the same size Number of columns of $A$ = number of rows of $B$