if x^2 + 9 = 10, then x would have been +- 1, both less than 5. ELIMINATED

if 3x - 8 = 10, then x = 6, also ELIMINATED.

But i think there is a typo, do you mean "f(x) = x^2 + 9 if x > -5", then the first case would work, and the answer would hence be 0.