Question 547542
What you wrote is  f(a+h)-f(a)/h={{{f(a+h)-f(a)/h}}}, with f(x)=5x^+4x-8.
I think that you may have meant (f(a+h)-f(a))/h={{{(f(a+h)-f(a))/h}}}, with {{{f(x)=5x^2+4x-8}}}
(That horizontal line is a very ink-efficient grouping symbol, it's equivalent to parenthesis around the expressions above it and below it).
If it's what I thought, {{{f(a+h)=5(a+h)^2+4(a+h)-8}}}, so
{{{f(a+h)=5a^2+5h^2+10ah+4a+4h-8=(5a^2+4a-8)+5h^2+(10a+4)h}}}
{{{f(a+h)-f(a)=5h^2+(10a+4)h}}}, and
 {{{(f(a+h)-f(a))/h=(5h^2+(10a+4)h)/h=5h+10a+4}}}