The distance of point A from origin is rA=√(√2−0)2+(√2−0)2=√4=2
The distance of point B from origin is rB=√(2−0)2+(0−0)2=√4=2
Potential at A due to charge Q=10−3μC is VA=Q4πϵ0rA=Q8πϵ0 as rA=2
and potential at B is VB=Q4πϵ0rB=Q8πϵ0 as rB=2
Potential difference between A and B =VA−VB=Q8πϵ0−Q8πϵ0=0 volt.