A force [itex]F_1=i-3j-2k[/itex] at the point [itex]-2i+9j[/itex], another force [itex]F_2=2i+j-3k[/itex] at the point [itex]-i+yj-k[/itex] and a third force [itex]F_3[/itex] are equivalent to zero. Find [itex]y[/itex] for this to be possible. Find [itex]F_3[/itex] and its line of action in this case I infer that "equivalent to zero" means this system of forces is in equilibrium. I understand that for equilibrium, the resultant force on the system has to be zero, and the sum of moments must also be zero. I understand how to calculate moments ([itex]M=r \times F[/itex]), but fail to see how I can deduce [itex]y[/itex] without any information on where [itex]F_3[/itex] acts. I have [itex]F_3=-3i+2j+5k[/itex] by working out [itex]F_1+F_2+F_3=0[/itex] but am stuck beyond that. Any help would be appreciated.