How to find the diagonal of this trapezoid?

Right tangential trapezoid, circle radius=r, the longer side of the trapezoid = 4r√3/3. Find the longer diagonal of the trapezoid (length).

I only know that a+b=c+d, so a+b=2r+4r√3/3 and the area is 2r^2 + 4r^2 √3 /3.

I'm sorry for my terrible maths terminology but I couldn't find any help in my language :(