Hard Question that I need help with!

This one is easier than it looks

Any "x" that makes a denominator 0  will not be in the domain

So

x^2 - 7  = 0

x^2  = 7        take both square roots

x = ±√7   but.....notice that the sum of these = 0 

x^3 - 8  = 0      the only real number that makes this = 0  is 2

x^4 - 9  = 0

x^4  = 9       take both 4th roots

x =  ± ⁴√9     and, like before, the sum of these two roots  = 0

So.....the sum of all the numbers not in the domain  is just 2