To draw the resulting force, complete a paralellogram
by drawing a line through each arrowhead parallel to
the other vector.  I'll draw them in green.  Notice
the angle marked  is so marked because it
is supplementary to the  angle, and .:



Now I'll draw in the resultant R in red:



In the lower triangle we have a case of SAS, so we
use the law of cosines first to find the magnitude
of the resultant R (the red vector).







Now we use the law of sines:











Edwin