We don't give full solutions here. When you show that you have made an attempt and show exactly where you are stuck, then we can help you further.
Here's some things to get you started:
1) This problem can be re-written as a^2 + a = 12 by substituting some bits for a (I'll let you figure that out).
2) Second order equations (of which this is one) have at most 2 solutions.
3) solve the equation in part (1)
4) express a in terms of x
5) the larger of the two solutions for x is your answer
note: it is possible for second order equations (quadratics) to have only a single solution.