The problem says to use vectors- so do that! Set up a coordinate system so that B is at the origin and the positive x-axis runs through C. Then C is (b, 0) for some number b, and A is (b/2, h) for some number h. Of course, D= (b/2, 0). You can calculate that the line AC is y= -2h/b(x)+ 2h and so can find the equation of the line and vector DE. Continue with that.