solving for the inverse of y = x² + 12x
Line 1:   x = y² + 12 x.   Â
Line 2   y² = x - 12 x. Â
Line 3 is y²= - 11 x.
Line 4 is y = √(- 11x)  for x ≥ 0.
The error lies in Line 1. All x's have to be switched with y's and vice versa for the method to work.
As such Line 1 should be  x = y² + 12y  NOT (x = y² + 12 x)
Since Line one is wrong, it messes with all the other lines.