Number 15, the last thing you would want on your burger king burger... but as it turns out... that may not be what you get...
for real tho you find the answer by looking at the ratio between the polygons.
Line DC on the left is similar to like JI on the right, you divide 3.2/5 to get .64 so you take any side on the left, multiply it by .64, and you got the missing side on the right. you do this to find Y by multiplying 4.5 (side AD) by .64 to get 2.88.
do the same thing but the other way around. divide 5/3.2 to find the ratio the other way around which is 1.5625, multiply it to x's corresponding side one right which is 1.28, so side X is 1.28 times 1.5625 which is 2.