This is a simple vector addition problem. If you draw this out on a sheet of paper you will quickly see that the 3 coulomb charge is being pulled greatly in the direction of the negative 5 coulomb charge.
To calculate the magnitude of this force to the left use the equation for Coulomb's Law: F = k x q1 x q2 / d^2. Where ‘k' equals 9.0 x 10^9 N m^2 / C^2.
You should get something close to 1.5 x10^11 N towards the negative 5 C charge.
You should know that your answer is so large because a coulomb is large. Most charges that you and I encounter in things like static charge from walking across a rug are on the order of microcoulombs, or millionth's of a coulomb.