Do it in 3 steps:

1. calculate f(x+h)
2. calculate f(x+h)-f(x)
3. calculate  

f(x) = 4x²-2x

1. f(x+h) = 4(x+h)²-2(x+h) = 

         4(x²+2hx+h²)-2x-2h =

         4x²+8hx+4h²-2x-2h  
  

2. f(x+h)-f(x) = (4x²+8hx+4h²-2x-2h)-(4x²-2x)

            = 4x²+8hx+4h²-2x-2h-4x²+2x   

            = 8hx+4h²-2h

            = h(8x+4h-2)

3. 

   

Edwin