15 + x + 2√(15 + x)(3 + x) + 3 + x = 36
2x + 18 + 2√(x + 15)(x + 3) = 36
x + 9 + √(x + 15)(x + 3) = 18
√(x + 15)(x + 3) = - x + 9
(x + 15)(x + 3)= (-x + 9)^2
x^2 + 18x + 45 = x^2 - 18x + 81
18x + 45 = - 18x + 81
18x + 18x = 81 - 45
36x = 36
x = 1
