Two charges of 20 μC and -10 μC are 6 cm apart. What is the electric potential at a point 4 cm away on the right bisector of the line joining the charges?
a) 2.1 x 10³ V
b) 4.7 x 10³ V
c) -5.2 x 10³ V
d) -2.8 x 10³ V