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Compute the torque about the origin of the gravitational force F = −mg acting on a particle of mass m located at r = x + y. (Use the following as necessary: m, g, x, and y). Torque = F (cross) r
Please help

Physics 
Elena,
If F=  (m•g)•j and r = (x) •I + (y) •j.
τ=  (m•g•x) [i x j] – (m•g•y)[j xj] = (m•g•x)•k.
i, j, k are the unit vectors of Cartesian coordinate system.