FYI - the length of the mid-segment of a trapezoid is equal to half the sum of the lengths of its bases.
the equation:
18 + 6x - 12 = 2(2x + 10)
18 + 6x - 12 = 4x + 20
6 + 6x = 4x + 20
6x = 4x + 14
2x = 14
x = 7
