The question is:

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.