What is the strength of the electric field 2.4 cm from a small plastic bead that has been charged to -9.0 nc ?

We can assimilate the small bead as a single point charge, with charge Q equal to

We want to find the electric field at a distance . The electric field of a single point charge is given by

where 
So, substituting, we find the electric field at the distance d:

And the negative sign means the field is directed towards the charge, since the charge is negative.