Using the definition of the difference quotient, we have
The goal here is to get the h term in the denominator to disappear.
First, we need to combine the terms in the numerator.
The common denominator is
Thus, the difference quotient becomes
Does this make sense?
By the way, to add on to what I told you in the other thread, when simplifying the difference quotient, the objective is to get the h term in the denominator to disappear (i.e. cancel out with something in the numerator of the difference quotient).