nothing wrong with that
you could just use the trig-vector notation
resultant = (10cos60,10sin60) + (20cos120,20sin120) + (30cos180,30sin180) + (40cos270,sin270)
= (5, 5√3) + (-10, 10√3) + (-30, 0) + (0, -40)
= ( -35, 5√3-40 ))
your sketch will confirm that