Wait, I'm sorry let me start from the beginning and rework this problem out.

Okay, I understand it now. The car is traveling faster than the truck between x equals 15, and 55. Which makes your domain or your base 40. The height reaches it's max when x equals 45. That's where your triangles are split in half. Between 45, and 55, your base is 10 and the height is 24. 10*24 is 240. Between 15, and 45 your base is 30, and the height is 24 again. 30*24 equals to 720. 720+240 is 960.

Oh wow I just figured it out!

The whole base of the triangle is 60, the max height is 36. 36*30 equals 1080. But we only want the car, we know the truck is traveling faster between 0-15, and 55-60. That makes two right triangles. We take the area of both triangles. 7.5*12 equals 90. That's the area of the first triangle. 2.5*12 equals 30. That's the area of the second triangle. The area of both Triangles of the truck becomes 120. The whole area was 1080. Take the area of the truck from the area of the whole triangle which leaves you with the area of just the car. 1080-120 equals to 960.