Using the Pythagorean theorem, the diagonal would be the hypotenuse of a triangle.
a^2 + b^2 = c^2
120^2 + 80^2 = c^2
14400 + 6400 = c^2
20,800 = c^2
c = SQRT(20800)
c = 40 sqrt(13) , This is the exact answer.
c = 144.22 yards, This is if you need it in yards ( May need to round off how the teacher wants it)
You do not say how the answer is to be written, so I have given you two ways.
