ES15 TwoDimensional Particle Equilibrium 
R =  For a particle to be in equilibrium, the resultant, or sum of forces added vectorially must be equal to zero. 
Consider the space diagram show here. 
The free body diagram (FBD) may be drawn if the forces on the box are to be found. 

Since the forces on the box are of interest here, the FBD of the vectors representing the weight of the box and the tensions supplied by the two persons is chosen. For equilibrium the FBD is referenced and forces in the two orthogonal directions are added as shown below. 

If F_{1} and F_{2}, the pull by the two people, are the unknowns, solving yields these terms... 