What have you tried so far? Have you subtracting an a from both sides, and then square both sides?
and so on ...
Therefore taking the series defined by and where
x can be seen as the limit (if it exists) of
If it converges, it is towards l such as which can be solved by a quadratic
At the end ... if I am not mistaken
It seems to be "minus" but I let you see why !!
All what I have written is to be checked ...