I didn't check your work so I'm assuming it's right. The resultant vector is in the third quadrant. Draw the two components so that you can make a right triangle. Now use the fact that tan(x) = (opposite)/(adjacent). Now compute the tan-inverse of this fraction and you'll get the angle. This is not the true angle, but the angle in respect to the quadrant so make sure to add 180 degrees onto it.
I hope you know what is a component i for component along x direction and J in the direction of the y axis , After you have resolved the forces in to its components you just need to find the direction of two forces along x and y axis ( ie; its components)
$\displaystyle tan \theta = \frac{b sin\alpha}{a+bcos\alpha}$
Where alpha is the angle(90 here) between two vectors(components in your case ,ie: a and b) and theta the angle of resultant