Decompose all vectors that are not cardinal directions
Ex = 400Ncos(-135) = -200sqrt(2) and Ey = 400Nsin(-135) = -200sqrt(2)
Now add all x vectors with E = pos. x and W= neg.x: 500 -400 - 200sqrt(2) = Rx
add all y vectors with N = pos. y and S = neg. y: -450 + 550 = 200sqrt(2) = Ry
|R| = sqrt(Rx2 + Ry2)
θ = arctan(Ry/Rx) (+180 if Rx is negative)