First let us sum the expressions
[tex]\begin{gathered} 3m+2n-6 \\ -6m+7n-4 \\ ----------- \\ -3m+9n-10 \end{gathered}[/tex]Hence, the sum of the expression is
[tex]-3m+9n-10[/tex]Now we put in n = -2 and m = 5 and evaluate the expression to get:
[tex]\begin{gathered} -3(5)+9(-2)-10=-15-18-10 \\ =-43 \end{gathered}[/tex]Hence, the expression evaluated at n = -2 and m = 5 gives
-43 and the answers are:
Part A; - 43
Part B : -3m + 9n - 10