3 charges are arranged along the x-axis.

q1 = -4.50 nC is located at x = 0.200m
q2 = +2.50 nC is located at x = -0.300m
q3 = is a positive character located at the origin.

What is the value of q3 for the net eletric force on it to be 4*10^-6 N?

I found the charge on q3 to be = 3 nC

Where along the x-axis can q3 be placed so that the net eletric force on it is 0, other than +/- infinity?

This is where I'm stuck.