Two charges q1 = 2e and q2 = -

Question

4e are placed on the x-axis at points (0; 0) and
(a; 0) respectively.
(a) What is the magnitude and direction of the electrostatic force on q2 due to
q1?
(b) Suppose a third charge q3 = -e is introduced at the location (b; 0) such that
b > a. What is the net force on q3 due to q1 and q2?
(c) If b < a, what should be the value of b as function of a so that the net
electrostatic force on q3 due to q1 and q2 exactly cancels out.