Ok so first you would want to multiply both sides by -5.
P(-5)=(m+n/-5)-5 (So now do the multiplication)
-5p= m+n (You got rid of the -5 because -5 multiplied by -5 is 1 and if you put one under m+n, it would still not change) Now, isolate the n.
-5p-m=n < That would be your answer :)
I really hope this helped :)