(i) Let C be the point on the line joining the two charges, where electric potential is zero, then
VC = 0

or VCA+VCB = 0 or VCA = - VCB
or

1

4πε0

qA

rCA

= -

1

4πε0

qB

rCB

or

5×10−8C

x×10−2m

= -

(−3×10−8C)

[(6−x)×10−2m]

or

5

x

=

3

16−x

or 80 – 5x = 3x or 80 = 8x
or x =

80

8

or x = 10 cm
So, electric potential is zero at distance of 10 cm from charge of 5 × 108 C on line joining the two charges between them.
If point C is not between the two charges, then
VCA+VCB = 0 or VCA = - VCB

or

1

4πε0

qQ

rCA

= -

1

4πε0

qB

rCB

or

5×10−8C

[(16+x)×10−2m]

= -

(−3×10−8C)

[x×10−2m]

5

16+x

=

3

x

or 5x = 48 + 3x or 2x = 48 or x = 24 cm
So, electric potential is also equal to zero at a distance of 24 cm from charge of −3×10−8 C and at a distance of (24 + 16) = 40 cm from charge of 5 × 10−8 C, on the side of charge of – 3 × 10−8 C