3 x (square root of) x + 3 = 15

3 x (square root of) x + 3 = 15

3 x (sqaure root of x) + 3 = 15
(Subtract 3 from both sides)

3 x (square root of x) = 12
(Divide both sides by 3)

(square root of x) = 4
(Square both sides)

x = 16

I hope this helps. :)