Find the Indicated Sum or Difference, If It Is Defined $\mathrm { A } - \mathrm { B }$

Find the indicated sum or difference, if it is defined.
-Compute the difference $\mathrm { A } - \mathrm { B }$ if $\mathrm { A } = \left[ \begin{array} { r r } - 1 & 0 \\ 6 & 3 \end{array} \right]$ and $\mathrm { B } = \left[ \begin{array} { r r } - 1 & 6 \\ 3 & 1 \end{array} \right]$ .

A) $\left[ \begin{array} { r r } 0 & - 6 \\ 3 & 2 \end{array} \right]$
B) $\left[ \begin{array} { r r } - 2 & 6 \\ 9 & 4 \end{array} \right]$
C) $[ - 1 ]$
D) $\left[ \begin{array} { r r } 0 & 6 \\ - 3 & - 2 \end{array} \right]$