(This question has already been answered, but both of the incorrect ones were deleted, so for future reference...)
Fig A is a scale image of Fig D. These two quadrilaterals are similar, and thus, the side lengths of corresponding sides are proportional. Set the proportion x/3 = 7.2/4 (you could alternatively write it as x/7.2 = 3/4 but for the simplicity's sake, and assuming anyone who has this problem would already know how to solve proportions like these...)
