buddy ...
you take 2 points a and b
there is no matter of that a is near the +ve end of battery and b is near the -ve end of battery
we need to find pot. diff between 2 batteries
V at a will be (+5) - (+2)=+3 volt
V at b will be (-5) - (-2)= -3 volt
thus Va-Vb=(3)-(-3)= 6 volt
and...