In the attached image, what is the potential difference across the capacitor?

Dear student 
There is DC supply , So the capcitor will open circuit at steady stste, 
So , there wil be no current in 3 microfarad and 3 ohm. 
So current will be = 15/5 = 3 A 
So current in BC wil be 1A 
Potential at C = 12V 
Potential at F will be = 0v 
So potential = 12V 
Hope thia will help