Note that, since f(x) = 10^x:
f(x + h) = 10^(x + h).
So, we see that:
[f(x + h) - 10^x]/h = [10^(x + h) - 10^x]/h = (10^x*10^h - 10^x)/h.
Factoring out 10^x gives:
[f(x + h) - 10^x]/h = (10^x*10^h - 10^x)/h = 10^x(10^h - 1)/h, as required.
I hope this helps!
