Alrighty so what it is asking you to do is to simply make variables. Let me break this down in an easier way for you. A variable is mainly just a fancy word that they use, it pretty much means a nickname how I use it. So for number fifteen in your work, we can simplify Monica into M to make it easier so:
S = Monica’s Sister
M = Monica
M = S+3
S = M-3
So what I did there is that I know that Monica is three inches taller than her sister so we know that her sister plus three inches would equal to her height.
And now for number fifteen in your work, I think it’s just asking to find the second one which it gives you
S = 2e - 3
But the second one would be
E = .5s + 1.5