Ineed 21 centimeters but i need to translate that into millimeters wht is the answer in milli eters

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.