Isaac Physics fields question help

https://isaacphysics.org/questions/oil_drop_multiple_fields?board=c85cba40-52a6-48e2-bcbf-51f5a6df74f5&stage=all

In a Millikan oil drop experiment the air is ionised by a beam of X-rays and a spherical oil drop of radius a, density ρ and charge +q is viewed through the microscope. The density of air is ρ0.

Part 3 of this question asks for a force equation when the oil drop is travelling through air at terminal velocity with downwards forces on the left and upwards forces on the right.

Surely this would be W=Fd+U where W is weight Fd is drag and U is upthrust?

In the context of this question that would give
4/3 pi a^3 ρ g = k a v + 4/3 pi a^3 ρ0 g.
I have tried this but its wrong. Any guidance on where my mistake is would be greatly appreciated! Thank you