How Is Charge q2 Calculated Using Electric Potential and Field Equations?

[nunos]

Have exam tomorrow. I know I should have studied before and more, but please help. If you know the answer, please take some time to reply. Thanks very much.

Homework Statement

There are two charges, q1 = +4.5nC, and q2, ?, which are at distances d1 = 2.5cm and d2 = 2.0cm, from a point which has the tension of 3.2kV. What is the value of q2?

Homework Equations

$$E = \frac{kQ}{r^2}$$, I think that's all, but I most probably am wrong.

The Attempt at a Solution

This is certainly wrong. I have not even used one of the data given. Anyone, that's the best I was able to come up with.

$$\frac{kq_1}{r_1^2} = \frac{kq_2}{r_2^2}$$

which gives $$q_2 = 1.28 * 10^{-6}$$ which is a wrong answer, since the correct one is supposed to be $$3.5nC$$.

 

[CompuChip]

You have not used that the potential difference U is 3.2 kV.
What does potential have to do with (difference in) electric field E?