The answer would be D. For you to find the solution you would need to look at the x from the coordinates.
Let number of senators = x, then:-
number of representatives = 5x - 25 so
x + 5x - 25 = 515
6x = 540
x = 90
number of reps = 515 - 90 = 425
so there are 90 senators and 425 representatives.
A prime expression is a math term. It is a expression that can't be divided by anything other than a 1.