Suppose $A$ is an $m\times n$ matrix and $B$ is an $n\times p$ matrix. Then for $1\leq i\leq m$, $1\leq j\leq p$, the individual entries of $AB$ are given by \begin{align*} \matrixentry{AB}{ij} &= \matrixentry{A}{i1}\matrixentry{B}{1j}+ \matrixentry{A}{i2}\matrixentry{B}{2j}+ \matrixentry{A}{i3}\matrixentry{B}{3j}+ \cdots+ \matrixentry{A}{in}\matrixentry{B}{nj}\\ &= \sum_{k=1}^{n}\matrixentry{A}{ik}\matrixentry{B}{kj} \end{align*}