You would subtract 15 on both sides and then you would have 2x=-14 then you divide both sides by 2 and x would equal -7
X^2 - 14x + 42 = 0
x^2 - 14x = -42
x^2 - 14x + 49 = 49 - 42
(x - 7)^2 = 7
x - 7 = (+-) sqrt 7
x = 7 (+-) sqrt 7
solutions are : x = 7 + sqrt 7 and x = 7 - sqrt 7