draw perpendicular lines from D and C to the line AB you'll find them equal, then find the length of those drawn lines by using the right angle triangle which was created by the drawn line(s).
The shorter base, from D to C, is 12 cm long. The distance between the bases of those two perpendiculars (lets call them E and F) is also 12 cm. That means the two pieces outside E and F total 36- 12= 24 cm. And looking at the right triangles formed at the ends by those perpendicular, "HL" tells you the two pieces are each length 12. Now, use the Pythagorean theorem.