Divide the rectangle into two right triangles, like in the photo
you now have two congruent right triangles with 7ft and 12ft long legs
Use the pythagorean theorem to find the missing diagonal length (aka hypotenuse)
7^2 + 12^2 = c^2
49 + 144 = c^2
c^2 = 193
Square root both sides to get rid of the ^2
c = 13.9 ft