Answer
If two matrices do not have the same order, they can not be added nor subtracted.
Work Step by Step
A matrix is a set of numbers, arranged into rows and columns, placed in brackets.
The numbers inside the brackets are called elements of the matrix.
A matrix of order $m\times n$ has $m$ rows and $n$ columns.
To add or subtract two matrices, they must have the same order. Otherwise, the sum/difference is not defined.
Matrices of the same order are added or subtracted by adding or subtracting corresponding elements.
Example:
$A= \left[\begin{array}{ll}
1 & 2\\
3 & 4\\
5 & 6
\end{array}\right]$ is a 3$\times$2 matrix, while
$B=\left[\begin{array}{lll}
1 & 2 & 3\\
4 & 5 & 6
\end{array}\right]$ is a 2$\times$3 matrix
so the sum $A+B$ and the difference $A-B$
are not defined.