√15-x+√3-x = 6
Heeeeeeeeeeeeelp

15 - x + 2√(15 - x)(3 - x) + 3 - x = 36 
18 - 2x + 2√(15 - x)(3 - x) - 36 = 0  /:2
9 - x + √(15 - x)(3 - x) - 18 = 0 
√(15 - x)(3 - x) = x + 9
(15 - x)(3 - x) = x^2 + 18x + 81
45 - 18x + x^2 = x^2 + 18x + 81
- 18x + 45 = 18x + 81
- 18x - 18x = 81 - 45
- 36x = 36
x = - 1

Проверка
√(15 + 1) + √(3 + 1) = 6
√16 + √4 = 6
4 + 2 = 6 
6  = 6