Answer: It is only +8 because it is an odd power, so you're only multiplying 3 numbers together and not 2 or 4 and so on. Any odd power is always only going to have 1 answer because if you try the other one then it comes out to be the opposite.
You only need the parenthesis if you want to plug it into a calculator because -7 in a calculator is taken as subtracting 7 while (-7) is taken as a negative 7. They are just building good habits early.