Electric Potential question help please

Two charges 3 x 10^-8 C and -2 x 10^-8 C are located 15 cm apart. At what point on the line joining the two charges is the electrical potential zero?

The sum of the electrical potentials at a single point in the line joining the charges is 0. This means:

Vsum = k(q/r) + k(q/r)
or
Vsum = k(q1/r1 + q2/r2)

The Vsum in this case has to be 0, q1 has to be the first charge, q2 the second, r1 has to be the distance from the first charge to the point of no potential and r2 the distance from the second charge to the point of no elec potential. If we use the variable d for r1, that means r2 is equal to 15-d.

Lets set Vsum to 0 and plug in all the values as well as the variables (excluding k for now):

0 = k( ( (3 x 10^-8 C) / d) + ( (-2 x 10^-8) / (15-d) ) )

you can divide k out and then get one of the two remaining terms equal to the other by subtracting one by the other:

(2 x 10^-8) / (15-d) = (3 x 10^-8 C) / d)

now you can cross multiply and solve for d:

d(2 x 10^-8) = (15-d)(3 x 10^-8)
d(2 x 10^-8) = (4.5 x 10^-7) - d(3 x 10^-8)
d(5 x 10^-8) = (4.5 x 10^-7)
d = 9 cm

This means that there is no electric potential 9 cms from the 3 x 10^-8 C charge.