Which of the following statements uses the commutative property of addition?

a 2 + (3 + 4) = (2 + 3) + 4
b 2 + (3 + 4) = (2 + 3) + (2 + 4)
c 2 + (3 + 4) = 2 + (4 + 3)

Arock was dropped from the top of a building. the function that describes the height after it was dropped is f(x)=64-16x^2. what are the zeros of the function and what do they mean?