If you're to use Same Opposite Always Positive (SOAP) method, you need to use the following rule:
a^{3} ± b^{3} = (a ± b) (a^{2} (opposite sign) ab (always positive) b^{2})
So, the formula that is given x^{3}+8 can be solved like this:
f(x + h) = 2(x + h) + 5
f(x + h) = 2x + 2h + 5
Therefore
f(x+h) - f(x) = 2x + 2h + 5 - 2x - 5 = 2h
[f(x+h) - f(x)]/h = 2h/h = 2