Question 265978
{{{drawing(400,250,-1,7,-1,4, 

rectangle(0,0,4,3), triangle(4,0,4+3*sqrt(3)/2,3/2,4,3),
locate(0,0,A), locate(4,0,B), locate(4+3sqrt(3)/2,3/2,E),
locate(4,3.3,C), locate(0,3.3,D) )}}} 

We are told that CD = 4cm, so we write 4cm on that side,
and we are told that DA is 3cm, so we write 3cm on that
side.

{{{drawing(400,250,-1,7,-1,4, 

rectangle(0,0,4,3), triangle(4,0,4+3*sqrt(3)/2,3/2,4,3),
locate(0,0,A), locate(4,0,B), locate(4+3sqrt(3)/2,3/2,E),
locate(4,3.3,C), locate(0,3.3,D), locate(-.5,1.6,3cm),
locate(2,3.3,4cm)


 )}}}

 Since DA and CB are opposit sides of rectangle ABCD, they
are equal in length, so let's also write 3cm on CB.

{{{drawing(400,250,-1,7,-1,4, 

rectangle(0,0,4,3), triangle(4,0,4+3*sqrt(3)/2,3/2,4,3),
locate(0,0,A), locate(4,0,B), locate(4+3sqrt(3)/2,3/2,E),
locate(4,3.3,C), locate(0,3.3,D), locate(-.5,1.6,3cm),
locate(2,3.3,4cm), locate(3.5,1.6,3cm) 


 )}}}

Now since we are given that triangle BCE is an equilateral
triangle, that means that all three of its sides have the same
length.  Since we know that side CB has length 3cm, then we
know that the other two sides BE and EC are also 3cm in length:

{{{drawing(400,250,-1,7,-1,4, 

rectangle(0,0,4,3), triangle(4,0,4+3*sqrt(3)/2,3/2,4,3),
locate(0,0,A), locate(4,0,B), locate(4+3sqrt(3)/2,3/2,E),
locate(4,3.3,C), locate(0,3.3,D), locate(-.5,1.6,3cm),
locate(2,3.3,4cm), locate(3.5,1.6,3cm),

locate(4.7,1,3cm), locate(4.7,2.8,3cm) 


 )}}}

Edwin