Careful! Remember that , so
Once you change your method to incorporate the two possible solutions, your method would work However, I would just have divided both sides of the equation by at the beginning:
Now take the square root of both sides, but remember that you should obtain two solutions as
You could choose to do this using the quadratic formula, but it would be unnecessary; you'd have to expand the brackets and then rearrange terms - this approach is far simpler!