In the given circuit with steady current the potential drop across the capacitor must be
In the steady state no current flows in the branch containing the capacitor .Thus the current say l flows in the branches containing R and 2R. Applying Kirchoff's second rule to the loop abcdefa
2V-I(2R)-IR-V=0
l=V3R
Potential drop across capacitor =2V-V-l(2R)=V−v3R× 2R
=V−2v3=v3