The extensions of the legs AB and CD of a trapezoid ABCD intersect at point E. Find the lengths of the altitude of △AED drawn to the base AD, if BC=7cm, AD=21cm, and the length of an altitude of a trapezoid is 3cm.

The extensions of the legs AB and CD of a trapezoid ABCD intersect at point E Find the lengths of the altitude of AED drawn to the base AD if BC7cm AD21cm and t class=