You need to find the ratio of the dilation so if you take corresponding sides, for example, 72 and divide by 8 you get 9. So, if you multiply each side of the triangle XYZ by 9, it should get the length of the sides on triangle ABC.
So a ratio of 8:3 means for every 8 cars that is 3 trucks so we have the number of trucks which is 21 so what i did to find the answer was see how many times 3 went into 21 which is 7 times then you would take the 7 and times it by 8 and get 56 so there are 56 cars.