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**A positively charged oil drop of mass 1.00 × 10−15 kg is placed in the region of a uniform electric field between two oppositely charged, horizontal plates. The drop is found to remain stationary under the influence of the Earth's gravitational field and the uniform electric field of 0.680 × 104 N/C. What is the magnitude of the charge on the drop?**

I am not sure where to start. I am given a mass so would I start with

U = -(G(m1)(m2))/r

Thanks in advance