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.