Forces and equilibrium — practice question

A box of small size with mass $40\,\text{kg}$ is pulled across a rough horizontal floor by three men. Two of them exert horizontal forces of magnitudes $100\,\text{N}$ and $120\,\text{N}$, directed at angles of $30^\circ$ and $60^\circ$ respectively to the positive $x$-direction. The third man exerts a horizontal force of magnitude $F\,\text{N}$ at an angle of $\alpha^\circ$ to the negative $x$-direction (see diagram). The combined effect of the three horizontal forces on the box is a resultant in the positive $x$-direction with magnitude $136\,\text{N}$.