Expand and simplify the forumla and then use apply Quadratic formula to solve it.
Substitute in the values, simplify, and you get your answer.
(I may have got it wrong... it might have been moved to the calculus section for a reason)
Are you familiar with how to complete the square? If so, then we rearrange your original equation,
Factor out the 3:
And complete the square:
Then some algebraic manipulation gets you (incoming wall-of-step-by-step-text):
From here, notice that:
Substituting that into the equation, we get:
Now, evaluate both solutions for x:
If you're not sure how to complete the square, I'm sure there are other methods to doing this, such as using the quadratic equation as DavidM suggested. The "tricky" part is recognising that can be expressed as a square in itself.