Question 841212
Notation is important.  You wrote this:
{{{f(x+h)-f(x)/ h}}}
but you really mean this:
{{{highlight_green(highlight_green((f(x+h)-f(x))/ h))}}}


Just form the expression using {{{f(x)=9x^2-2}}} and simplify.
{{{(9(x+h)^2-2-(9x^2-2))/h}}}
{{{(9(x^2+2xh+h^2)-2-(9x^2-2))/h}}}
{{{(9x^2+18xh+9h^2-2-9x^2+2)/h}}}----look carefully in numerator for additive inverses
{{{(18xh+9h^2)/h}}}
{{{highlight(18x+9h)}}}