Hello, I was wondering if some knows how to put a box around all the 4 entries marked with * in the 3x3 matrix below:
\begin{pmatrix}
a* & b* & c\\
d* & e* & f
\end{pmatrix}
I am trying to put one box around all 4 entries, and not 4 separate boxes around each of the entries. Thank you in advance.

