Answer
Two matrices can not be multiplied if the number of columns of the first matrix does not equal the number of rows of the second.
(example provided in the step-by-step section)
Work Step by Step
To multiply two matrices,
the first must have as many columns as the other has rows.
The product of an $m\times n$ matrix $A$ and an $n\times p$ matrix $B$
is an $m\times p$ matrix $AB$.
Example:
A$=\left[\begin{array}{lll}
1 & 2 & 3\\
4 & 5 & 6
\end{array}\right]$ is a 2$\times$3 matrix
B$= \left[\begin{array}{l}
1\\
3\\
5
\end{array}\right]$ is a 3$\times$1 matrix, so
$AB$ is defined, and is a 2$\times$1 matrix.
(the number of columns in A is 3,
the number of rows in B is 3)
BUT
$BA$ is not defined
(the number of columns in B is 1,
the number of rows in A is 2)
