I assume you mean the equation
√(2x) = x - 4
Take the square of both sides. Note that √(2x) is a non-negative number, so any solution we find must also satisfy x - 4 ≥ 0, or x ≥ 4.
(√(2x))² = (x - 4)²
2x = x² - 8x + 16
x² - 10x + 16 = 0
(x - 8) (x - 2) = 0
x - 8 = 0 or x - 2 = 0
x = 8 or x = 2
If x = 2, then x ≥ 4 is false. The only solution is then x = 8.
