What I've done-

How do I proceed further from here?

 

Walk along any path from A to B and determine Vab (or Vba, depending on how Voc is defined, which isn't specified).

 

Walk along any path from A to B and determine Vab (or Vba, depending on how Voc is defined, which isn't specified).

Voc is voltage across open circuit

 

Voc is voltage across open circuit

That isn't enough. Voltages and currents have both location AND polarity. Is Voc the voltage at 'a' relative to 'b' (also known as Vab) or is it the voltage at 'b' relative to 'a' (also known as Vba). These are NOT the same thing -- the difference is extremely significant. Connect your car battery backwards and you will quickly discover this to be the case.