Solution
= Solution
The product of a $3\times n$ <matrix> with another $3\times n$ <matrix> requires its inner dimensions $n$ and three to match. Thus $\boxed{n=3}$ is necessary and sufficient for $M^2$ to be defined.
= Solution
The product of a $3\times n$ <matrix> with another $3\times n$ <matrix> requires its inner dimensions $n$ and three to match. Thus $\boxed{n=3}$ is necessary and sufficient for $M^2$ to be defined.