Let
{\color{blue}A \in M_{M \times N}} and
{\color{red}B \in M_{K \times L}}.
Then the product {\color{blue}A} \cdot {\color{red}B} is
M \times L - matrix
not defined.
M \times N - matrix.K \times L - matrix.N \times K - matrix.L \times K - matrix.M \times N - matrix.K \times L - matrix.N \times K - matrix.L \times K - matrix.L \times M - matrix.
To define the product {\color{blue}A} \cdot {\color{red}B}
the number of columns of {\color{blue}A}
must equal the number of rows of {\color{red}B}.
This is the case here, and the product has M rows
and L columns.
Therefore {\color{blue}A} \cdot {\color{red}B} \in M_{M \times L}.
To define the product {\color{blue}A} \cdot {\color{red}B}
the number of columns of {\color{blue}A}
must equal the number of rows of {\color{red}B}.
This is not the case here.