The problem is simply

f(x) = x^(3/2)

using (f(x+h) - f(x)) / h and multiply with the conjugate I get

(x+h)^3 - x^3

/

h(x+h)^(3/2) + z^(3/2)

How do I get forward from this? I have been trying for hours now, but the algebra gets really ugly. Is there an easier way to reach the answer?

Sorry I don't know how to write formulas in LaTex