Since the exponent is 3, and in the parentheses is x+y, it's basically
(x+y)(x+y)(x+y)
You write out x+y 3 times since the exponent is 3.
If the exponent was 4, you would write out x+y 4 times and so on.
The 3 gives value to the x+y and how many to use or calculate before getting your answer.
