If x is the x-coordinate of P, obviously x must be positive (There is no
possibility of potentials due to the two charges adding up to zero for
x < 0 ) If x lies between O and A, we have
41
3 10
10
152 10
10
0
0
8
2
8
2
πε
××
−
−×
×
=




–
–
–
–
(
)
x
x
where x is in cm That is,
3
2
0
15
x
x
−
=
−
which gives x  =  9 cm