3x^2 - 18x + 15
3x^2 - 18x = -15...divide by 3
x^2 - 6x = - 5...take half of -6, and square it...and add to both sides
x^2 - 6x + 9 = -5 + 9
x^2 - 6x + 9 = 4
(x - 3)^2 = 4
x - 3 = (+-) sqrt 4
x - 3 = (+-) 2
x = 3 (+-) 2
x = 3 + 2 = 5
x = 3 - 2 = 1
so x = 5 and x = 1
