Find the magnitude of a fourth force on the stone that will make the vector sum of the four forces zero.

100cos30° + 80cos120° + 40cos233° + Dx = 0

Dx = - 22.53 N

y components:

100sin30° + 80sin120° + 40sin233° + Dy = 0

Dy = - 87.34 N

magnitude of D:

sqrt[ (-22.53) ² + (-87.34) ²]

90.2 N

direction of D:

arctan[ (-87.34) / (-22.53) ]

75.5° ref, but since Dx and Dy are both negative, we know this vector is in QIV:

360 - 75.5° = 284.5°