Solving functions help

You're given a function f(x)=(sqrtx^2 +6x)
1) Solve the equation: f(x)=3x-2
2) Solve the inequality: f^2(x) < f(-6)
Results: 1 - x=2; 2 -x E(-6;0)

How should I solve these? In a previous exercise I was given "x" thus I just had to replace x in the function, but in this case that doesn't seem to work.
How exactly should I proceed?

Thanks in advance!

1) f(x) = 3x - 2
√(x^2 + 6x) = 3x - 2
x^2 + 6x = (3x - 2)^2
x^2 + 6x = 9x^2 - 12x + 4
8x^2 - 18x + 4 = 0
(x - 2)(4x - 1) = 0
x = 2, x = 1/4

2) f²(x) < f(-6)
√(√(x² + 6x)² + 6x) < √((-6)² + 6(-6))
√(x² + 6x+6x) < √(36-36)
x² + 12x < 0
x(x+12) < 0
since a=1, the graph is a u-shaped parabola. The answer -12