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A particle is acted upon by the forces
F1=2ˆi+aˆj−3k
F2=5ˆi+cˆj−bk
F3=bˆi+5ˆj−7k
F4=cˆi+6ˆj−ak.

Find the values of the constants a, b, c in order that the particle will be in equilibrium.