Question 295694
sqrt(12 - 5x} = 5x

Square both sides.

[sqrt{12 - 5x}]^2 = (5x)^2

12 - 5x = 25x^2

Put everything on one side and equate to zero.

25x^2 + 5x - 12 = 0

You now have a quadratic equation.

Can you take it from here?