I was unable to get a camera to work, so You'll have to read carfully.
1.Construct a trapezoid with the description of the problem.
2.Name the verticies starting from the top left side, going clockwise, A,B,C and D respectively.
3.Draw altitudes going down from A and B. Name the points where they intersect DC F and E, F is on the left.
4.Let side \(\overline{AB}\) be a, and \(\overline{DC}\) be b. \(\overline{FE}\) is a, and \(\overline{EC}\) = \(\overline{DF}\) = \(\frac{b-a}{2}\).
5.Let the intersection of diagonals be O.
6.\(\triangle DOC\) is a 45-45-90 triangle.
See if you can work it out from there.