Q.Evaluate: $\begin{bmatrix} 2 & -1 & 3 \end{bmatrix} \begin{bmatrix} 4 \\ 3 \\ 1 \end{bmatrix}$

Two matrices $A_{m\times n}$ and $B_{n\times p}$ can be multiplied — "are conformable for the product $AB$" — exactly when the number of columns of $A$ matches the number of rows of $B$; the product $AB$ then has order $m\times p$, with each entry $C_{ij}=\sum_k a_{ik}b_{kj}$ formed by "dotting" row $i$ of $A$ with column $j$ of $B$. Matrix multiplication behaves very differently from ordinary number multiplication: it is generally not commutative ($AB\ne BA$, and one product may not even be defined while the other is); it is associative ($(AB)C=A(BC)$) and distributes over addition ($A(B+C)=AB+AC$); multiplying by the identity matrix $I$ leaves a matrix unchanged ($AI=IA=A$); and — sharply unlike numbers — the product of two non-zero matrices can equal the zero matrix. Because $AB\ne BA$, algebraic identities that are automatic for numbers (like $(A+B)^2=A^2+2AB+B^2$) pick up extra cross-terms for matrices: $(A+B)^2=A^2+AB+BA+B^2$. Matrix multiplication is also how real data — quantities and prices, or students and fees — gets combined in one shot, which is why word problems in this chapter (bookseller totals, sports-academy fees, purchase amounts) are all set up as a single matrix product.