Solve the equation
x-1= square root 3x+15
x-1 = √(3x+15)
square both sides:
(x-1)^2 = 3x+15
x^2 - 2x + 1 = 3x+15
x^2 - 5x - 14 = 0
(x-7)(x+2) = 0
x = 7 or -2
check:
7-1 = √(3*7+15)
6 = √36
ok
√ is positive, so -2 is a spurious solution. It is the solution to x-1 = -√(3x+15)