I would plug Equations 2 and 3 into 4 and show that you get Equation 1, for an appropriate choice of N. Then, since all your steps are reversible, you've shown that which you were asked to show.
I am having a lot of difficulty figuring this one out. The goal is to show that equations 1, 2, and 3, give the final simplified equation 4. I have always been more of a calculus person, and have trouble with these types of algebra problems.
So far I have tried multiplying both S and Sigma together and attempted to put ga into the equation, which didn't really get me anywhere. Any help is appreciated!