1. Check the picture at the bottom.
2. ABCD is a right trapezoid so the diagonals have equal length. Lets find diagonal BD.
3. In triangle ABD we are given the length AB=7, AD=5 and m(A)=80°.
4. We use the cosine law:
5. Cosine of 80 can be found with a scientific calculator or in the internet as cos80=0,17
6.
square root of BD=7.88, so the answer is 8 feet.