Prealgebra (7th Edition)

perimeter is $40\frac{2}{3}$ ft
The combined lengths of the 2 segments at the top equals the length of the bottom side. $3x+7=12$ $3x+7-7=12-7$ $3x=5$ $x=\frac{5}{3}$ The perimeter is: $12+12+5(\frac{5}{3})+5(\frac{5}{3})= 24+\frac{50}{3}=24+16\frac{2}{3}=40\frac{2}{3}$