When solving the square of a quadratic in the form , first you put into brackets, .
So in the case of your question you get . However, if you multiply out ,you find that you get an extra 9 in your equation, to compensate for this you also place a -9 in your equation, giving you .
A little bit of rearranging should give you , which in turn leads to .
I presume you can continue from here to find the two solutions.