You have a system of equations going on here. The first one is already set equal to a. So let's take that part of the equation, the p + 15, and sub it in for the a in the other equation: 5(p + 15) - 525 = 2p. Now what we have is an equation to solve that is only in terms of p, not p and a. We cannot solve an equation with 2 unknowns. Making that substitution eliminates the a, the second unknown. Now distribute the 5 into the parenthesis to get 5p + 75 - 525= 2p. Move the 2p over to the left and move the other terms over to the right and combine all the like terms to get 3p = 450. Dividing both sides by 3 gives you that p = 150. Now take that p value and sub it into the p + 15 = a to get 150 + 15 = a and a = 165
