Determine an expression, in simplified form, for the slope of the secant PQ

a) , where

Now, i know which formula to use, which is

i have the solution to this problem, but don't understand where the numbers are coming from so it's useless to me. i don't understand how to plug the numbers into this formula

for the first step it is:

then simply simplifying to get

3h+6

can someone please explain this to me?