Because AB, EF and CD are perpendicular to the ground, BD, AD, AD AD BC intersect at E.

So: triangular DEF is similar to triangular DAB.

Similarly, the triangle BEF is similar to the triangle BCD.

So: EF/AB=FD/BD=3/4.

Let FD=3x, then BD=4x, BF = X.

Because: triangle BEF is similar to triangle BCD.

So: EF/CD=BF/BD= 1/4.

Because: EF=3

Then: CD= 12m.