Net electric force on charge from three other charges on a line

[RichMarrem]

Homework Statement

A, B and C have charges of -2.6, 8.5, and 7.5 microC respectively. These charges are at the points 2.5, 7 and 2 on a straight line where all distances are measured in millimeters from the zero point. What force does a -7.5 microC charge feel if it is at the point 1.2 mm?

Homework Equations

F = k(q1q2)/r^2

The Attempt at a Solution

I tried making free body diagrams and then using those to make net force equations but I was unsure how to find the force from the equations.

 

[ehild]

What force acts between two point charges? Show your work. ehild

 

[RichMarrem]

Would it be F = k(q1q2)/r^2 ?

so between A and the fourth charge,
F = k(2.6*7.5)/(1.3)^2

and so on between B, C and the fourth charge