m + n = 5
Alright, so unFOILing the numerator, you'd get (x+5)(x-3). However, since (x-3) is not there in the second fraction, we know that the denominator must also have had (x-3), causing them to cancel.
Now that we know one of the parts to the denominator is (x-3), the second fraction tells us the other: (x-4).
Thus, FOILing (x-3)(x-4) gives us x^2 - 7x + 12. So, m = -7 and n = 12, and just add them up.