Rewrite completing the sqaure 3x^2-18x+15

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