How can you solve an electric force problem with vectors?

[ASiCaRiO]

Homework Statement

http://imageshack.us/a/img145/3799/img0302ok.jpg

Homework Equations

f=k*q1q2/r^2

The Attempt at a Solution

i just don't know how to start i just need a hint and i will do the rest

 

[ehild]

Draw vectors representing the individual forces the other charges exerte on q_o. The vector sum of these vectors has to be zero.

ehild