x=18. Similar meaning they are proportional.
Quadrilateral 1 : 3, 9, 12 , x
Quadrilateral 2 : 2.25, 6.75, 9, 13.5
So we ask our self how is it being scaled. So I took 3 and 2.25 because those represent the same side and divided them to find the scale factor to see how you get from Quadrilateral 2 lengths to quadrilateral 1. You get 1.3333 or 4/3. so we are trying to find x which was a part of the bigger quadrilateral. This means x should be greater than the corresponding side on the other quadrilateral which is 13.5. So we take our 4/3 and multiply it by 13.5 to find x.
Quadrilateral 1: 3, 9, 12, 18
Quadrilateral 2: 2.25, 6.75, 9, 13.5
Now if to check if you are right 18/13.5 should equal (4/3) which was our scale factor to get from Quadrilateral 2 lengths to the corresponding lengths of Quadrilateral 1. If we want to scale down from Quadrilateral 1 to Quadrilateral 2 just divide the length by (4/3) or multiply by the reciprocal which is (3/4)
