Net electric forces on a point charge

[Colts]

Homework Statement

In the figure, what is the force on charge Q₃?
Q₁ = 5×10^-6C, Q₂ = -3×10^-6C, Q₃ = 4×10^-6C
http://www.usi.edu/science/physics/pickett/206/4p2f1.jpg

Homework Equations

Coulomb's law

The Attempt at a Solution

I thought I could just find the charges on q1 and 3 and then on 2 and 3 and add them but I don't think that is right. Can you just add electrical forces?

 

[berkeman]

Homework Statement

In the figure, what is the force on charge Q₃?
Q₁ = 5×10^-6C, Q₂ = -3×10^-6C, Q₃ = 4×10^-6C
http://www.usi.edu/science/physics/pickett/206/4p2f1.jpg

Homework Equations

Coulomb's law

The Attempt at a Solution

I thought I could just find the charges on q1 and 3 and then on 2 and 3 and add them but I don't think that is right. Can you just add electrical forces?

Yes, use Coulomb's law and sum the 2 forces on Q3 to get the total force on Q3.