The trapezoid in question is an isosceles trapezoid with base lengths of 12 ft and 8 ft and both sides equal to 7 ft.
____8 ft___
/ \
7 ft / \ 7 ft
/____________\
12 ft
The perimeter of a single trapezoid is:
P = 8 ft + 7 ft + 7 ft + 12 ft = 34 ft
Andy will need 34 ft of stones per trapezoid. So in total:
3 x 34 ft = 102 ft
Andy will need 102 ft of stones in total.