Two point charges (+9.0 nC and -9 nC) are located 8.00 cm apart. Let U = 0 when all of the charges are separated by infinite distances. What is...

You have not mentioned where the point b is where the
charge q = -3.2 nC is placed.

For a charge q placed at a
distance r from a charge Q, the potential energy is equal to U = kQq/r, where k is the
Coulomb's constant.

In your question if q is at a distance
Ra from the 9 nC charge and a distance Rb from the -9 nC charge, the potential energy of
the charge q would be:

k*9*10^-9*(-3.2*10^-9)/Ra +
k*(-9*10^-9)*(-3.2*10^-9)/Rb

= -k(28.8*10^-18)/Ra +
k(28.8*10^-18)/Rb

= 28.8*10^-18*k*[1/Rb -
1/Ra]

This would be equal to 0 if Ra = Rb and the charge
does not have any potential energy due to the oppositely charged particles. For all
other values of Ra and Rb, the charge has a net potential
energy.