In a particular millikan oil-drop apparatus the plates are

Question

Here is the problem which I am struggling on, please help me!

Problem: In a particular Millikan oil-drop apparatus, the plates are 2.25cm apart. The oil used has a density of 0.835g/cm³, and the atomizer that sprays the oil drops produces drops of diameter 1.00x10^-3mm.

Part I: What strength of electric field is needed to hold such a drop stationary against gravity if the drop contains five excess electrons?

Part II: What should be the potential difference across the plates to produce this electric field?

Part III: If another drop of the same oil requires a plate potential of 66.9V to hold it stationary, how many excess electrons did it contain?

I need tutor help to solve the above problem.