The third force C is in the opposite direction of the sum of the first two forces, A+B, but the magnitude is the same as A+B
Since A & B are at right angles we can just use Pythagoras and trig:
A+B magnitude = sqrt(6^2 + 3^2) = 6.71
A+B angle = arctan(6/3) deg north of east = 63.4 deg (first quadrant)
So C magnitude = 6.71 (same as A+B)
[C angle = 63.4 deg south of west (third quadrant) = -180+63.4 deg polar = -116.6 deg
