Net current =net emf/net resistance
Now
Net emf = 8V-4V =4v.....as cells are in series but oppositely connected
Net internal resistance= 0.5ohm + 0.1 ohm= 0.6 ohm
And external resistance comes out to be 6.5ohm
Now current= 4V/6.5+0.6 ohm= 4/7.1A=0.56A