Finding net force on object with more than one force

[Planefreak]

Homework Statement

Three 8kg spheres are located at (0, 1.7), (1.7, 1.7), and (1.7, 0) named A, B, and C respectively. What is the net gravitational force acting on sphere A?

Homework Equations

F = GMm/r²

The Attempt at a Solution

So far I've tried this several ways with no success. I'm not really sure where to begin. Since there is only one acting in the vertical direction I started there. I found the amount of force that the two had towards each other. Then I did some math to determine how much of that is downward. I came up with 3.1388e-10 which is incorrect. I tried the other one in a similar fashion. I found both of the forces then added the two together. Not directly but finding how much is being pulled horizontally and adding to the other. I'm almost 100% sure that I am forgetting something or just messed up somewhere along the way.

 

[LowlyPion]

They are vectors aren't they?

G1 = ... i + ... j

G2 = ... i + ... j

G1 + G2 = ... ?