posted by Mischa .
Three point charges, q1, q2, and q3, lie along the x-axis at x=0, x=3 cm, and x=5 cm, respectively. Calculate the magnitude and direction of the electric force on each of the three points when q1=+6.0 muC, q2=+1.5 muC, and q3=-2.0 muC.
I would add the forces. Watch direction. Draw a figure, label the directions (like charges repel, unlike attract) at each point. That way, you wont get the signs wrong.
Example: at q2, the right charge pulls the q2 to right (call that positive), and the left charge repels it to the right (again positive) so in this case the forces add to the right.