Physics education research, electronic materials, and the musings of Christopher Moore, Ph.D.

Hi, Im working on the following question.
A 64 µC charge is located 30cm to the left of a 16 µC charge. What is the resultant force on a -12 µC charge positioned exactly 50mm below the 16 µC charge?

I set it up like this.
a                   .3m                b
64 µC ---------------------------16 µC
                                             i
                                             i
                                             i     .05m
                                             i
                                           -12 µC
                                             c
So I tried to solve using the vector components as follows
Fac=9.0e9(65x10e-6 * 12x10e-6)/(.3^2+.05^2) = 74.72C
Fbc = 9.0e9(16x10e-6 * 12x10e-6)/(.05^2) = 691.2C

I'm not sure where to go from here...I've tried to solve is a number of times but cant get the correct answer. I think when i set the problem up to solve for the vectors I'm doing it wrong.