Question

What are the excluded values? m+5/mn+3m

Answer 1

values that are excluded from the domain of a rational expression are values that make the denominator 0, since if that's so, the rational will be undefined.  That happens when the denominator is zero out, let's do so

$\bf mn+3m=0\implies m(n+3)=0\implies \begin{cases} m=0\\ n=-3 \end{cases}$

so, if ever m = 0, the denominator will become 0 and the rational becomes undefined, and whenever n = -3, the same will happen to the rational, thus those values are excluded.