First find the distance from charge a to charge b:
Now find the force exerted by charge b on charge a:
The negative sign in the answer means that there is an attraction (unlike charges) so the direction is straight down, towards charge b.
Now for the force exerted by charge c on charge a:
The positive sign in the answer means that there is a repulsion (like charges) which means that the direction is away from charge c so 30 degrees above horizontal to the left.
Now the resultant vector of these two vectors must be found. I will use the law of cosines, but you could also break up the vecors into their x and y components and just add them up:
Now we have the magnitude, we just need to find the angle. I use the law of sines:
The force is 
exerted at 
west of south.
Note: This problem also shows the nature of the coulomb. One coulomb is a huge charge. Usually you deal in microcoulombs, for as you can see, putting such charges at this distance would cause a HUGE force.