Question 960694
You need all of the proper grouping parentheses.


This is not what you want:  {{{f(x+h)-f(x)/h}}}


You want THIS:  {{{(f(x+h)-f(x))/h}}}


Now just substitute for f(x).


{{{((7(x+h)-4)-(7x-4))/h}}}

Continue from that.  The result will be 7.