The charges and coordinates of two charged particles held fixed in the xy plane are: q1 = +3.3 µc, x1 = 3.5 cm, y1 = 0.50 cm, and q2 = -4.0 µc, x2 = -2.0 cm, y2 = 1.5 cm. (a) find the magnitude and direction of the electrostatic force on q2

1) First of all, we need to find the distance between the two charges. Their distance on the xy plane is

substituting the coordinates of the two charges, we get


2) Then, we can calculate the electrostatic force between the two charges  and , which is given by

where  is the Coulomb's constant.
Substituting numbers, we get 

and the negative sign means the force between the two charges is attractive, because the two charges have opposite sign.