Lining up the trapezoids you need to notice that you have measurements for the LEFT side of both trapezoids. These measurements will be used to set up the ratio of small to large. 8/12 = 2/3 is the ratio of small trapezoid to large trapezoid.
To find a: 8/12 = 12/a; cross multiply; 8a = 144, divide both sides by 8; a = 18
To find b: 8/12 = b/6; cross multiply; 12b = 48, divide both sides by 12; b = 4
To find c: 8/12 =c/15; cross multiply; 12c =120, divide both sides by 12; c = 10
Instead of using 8/12 you could have used the reduced version, 2/3.