Answer :
Because base angles on an isosceles trapezoid are equal you set up this equation
7x - 12 = 5x + 6
-5x+12 -5x +12
2x = 18
/2 /2
x = 9
7x - 12 = 5x + 6
-5x+12 -5x +12
2x = 18
/2 /2
x = 9