ok im not sure why i keep getting the answer an 3 to this problem, im hoping someone can show me where im going wrong....

ok so here goes...

we have a triangle ABC, where AB = 6 and AD = 3. what is the length of CD?

before i go on, BD is a perpendicular bisector to AC.

so anyways i found the length of BD to be 5. now im not sure where to go from here...i tried to use triangles ABD and BDC but this isnt right...