Question 823666
Proper grouping symbols are important.
{{{highlight((f(x+h)-f(x))/h)}}}


Using your function,
(x^2-7x+4),


{{{(((x+h)^2-7(x+h)+4)-(x^2-7x+4))/h}}}
{{{(x^2+2hx+h^2-7x-7h+4-x^2+7x-4)/h}}}
{{{(2hx+h^2-7h)/h}}}
If nothing was missed, then obtain
{{{highlight(2x+h-7)}}}