Quadrilateral has four sides. Not necessarily a rectangle, just four sides.
They are similar, so the sides of one of them and the other one are in proportion.
We have to find the proportion. Notice that the 12 feet side and the 18 feet side must be the one in common. 3 longest ones ... one is missing, then, two shortest ones ... two are missing, but they must share that one:
The ratio is the 18:12 or 3:2.
Now we can complete both quadrilaterals:
ABCD: 24, 16, 12, ?
EFGH: ?, ?, 18, 9
ABCD: 24, 16, 12, 6
EFGH: 36, 24, 18, 9
24 is the answer!!