g(x)=2x2+5
The difference quotient is the slope of the secant line connecting two points, (x,g(x)) and (x+h,g(x+h)), along the curve g(x):
[g(x+h)-g(x)]/h
= [2(x+h)2 + 5 - (2x2+5)]/h
= [2x2 + 4xh + 2h2 + 5 - 2x2 - 5]/h
= [4xh + 2h2]/h
= 4x + 2h