Suppose the length of is .
Then, using the formula, write down the areas of 's and . (Hint: the area of .)
Then consider these two triangles as though their bases lay along the line . So they have a common height. So the ratio of their areas is the same as the ratio of the lengths of their bases which is C" alt="BDC" />.
Then use your first two areas, and you're there.
(Incidentally, although you're told that , it doesn't have to be, provided it is bisected by .)