Answer: Choice A) 15 feet
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x = length of fourth side in quadrilateral ABCD
Have a look at the attached image I've posted. I made a chart that lines up the side lengths from largest to smallest. Note how the first two items in the bottom row are blank. We don't know these values and we don't need to find out in order to find x.
Based on the last two columns of that table, we can form the proportion below, which is then used to isolate x.
30/12 = x/6
30*6 = 12*x ... cross multiply
180 = 12*x
12*x = 180
x = 180/12 ... divide both sides by 12
x = 15
