sale price / precentage * 100 = real price
x^2+15x-2
x^2+15x-2 = (x+a)(x+b)
a plus b makes -2
a times b makes 15
a = -5 and b = -3
if (x+a)(x+b) = 0
either (x+a) = 0 or (x+b) = 0
so (x+-5) = 0 or (x+-3) = 0
or x is either 5 or 3
