Since the triangles are similar, the following is true:
20 x+6 ------- = ------- 2x -6 36
Then (20)(36) = (x+6)(2)(x-3) = 2[x^2+3x-18], or
720 = 2[x^2+3x-18], or 360 = x^2 + 3x - 18.
Then x^2 + 3x - 18 - 360 = 0. This is a quadratic equation which can be solved using the quadratic formula, among other approaches. Please apply the quadratic formula here, using a=1, b=3 and c= -378, to find x.
≈5.61