If A $=\begin{bmatrix} 2 & 1\\1 & 4\\-1 & 6\\0 & 7\end{bmatrix}$ and $B=\begin{bmatrix} 1 & 2 & 3 & 0\\2 & -1 & 6 & 7\end{bmatrix}$ then

AB and BA both are defined.

order (A) = 4 × 2

order (B) = 2 × 4

so in A × B (No. of columns in A = No. of rows in B)

Therefore, AB is defined.

No. of columns in B = No. of rows in A

BA also defined.