How to solve .
The approach you're trying to use is completing the square. You've done well to get to here:
Unfortunately you then proceed to undo all this, effectively going in a circle and arriving back to .
Here is what you must do. Divide both sides of by 9:
which is clearly a difference of two squares.
From your previous posts I know that you (should) know how to factorise this and hence get the solution.