suppose there is an U shaped wire, in which BD is another wire which is movable threough it.
suppose you have dipped it into soap solution and stretched BD by x distance.there is always a force acting on BD ie l length due to surface tension.
work done in doing so=forcex dist
=Tlx as surface tension T=F/l
this work done is equal to the increase in energy / surface energy.
so T lx=increase in surface energy
so T=increase in surface energy/lx
=increase in surface energy/increase in area