Total electric force on a particle in an equilatteral triangle

[poipoipoi]

Three charged particles are located at the corners of an equilateral triangle. Calculate the total electric force on the 7.00µC charge.
The three charges are: 7.00µC, 2.00µC, and -4.00µC and they are all 0.500 meters apart. Here's a graphic of the layout:
http://gyazo.com/34786cc28a37e5b918db63a7e9d6f8c2.png

I know I have to use Coulomb's law but am having trouble coming up with the answer.

 

[N-Gin]

I know I have to use Coulomb's law but am having trouble coming up with the answer.

Yes, use Coulomb's law to calculate the forces. Just don't forget to treat them as vectors when calculating the net force. There's some basic trigonometry involved so it shouldn't be a problem.