We know that the temperature /was/ -3, so it was 3 under zero. It has /risen/ by 8, meaning we have added 8 degrees to whatever the starting temperature was.
We can find out how much the temperature is right now: it was -3, went through -2, -1, 0, 1, 2, 3, 4, and now its at 5 degrees. Thats risen by 8 degrees, right? And -3+8 is the same as 8-3 (commutative property of addition, since we basically did 8+(-3)) which equals 5, so we know that we got the correct answer. :)
