Thanks I don't understand how to do this problem.



We are told that DA is 3 cm. So we put 3 on DA:



Now we know that CB and DA have the same length because
they are opposite sides of a rectangle. And since DA has
length 3 cm. so does CB,  So we put a 3 on CB:


 
We are told that triangle BCE is equilateral, so all three
sides of it have the same length.  And since CB is a side
of the equilateral triangle BCE, both CE and BE have the
same length as CB. Since CB has length 3 cm, so doe BE, so
we put 3 on BE and CE.



We didn't even need the fact that the length of CD is 4.

The answer is "the length of BE is 3 cm."

Edwin