If f(x)= 10^x, show that f(x+h)-f(x)/h=10^x(10^h-1/h

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!