First, you have to write the equations that express x, y and z in terms of θ, φ and r. Then you write i, j and k in terms of the unit vectors [itex] e_\theta, e_\phi[/itex] and [itex] e_r [/itex]. Then you just substitute.
First, you have to write the equations that express x, y and z in terms of θ, φ and r. Then you write i, j and k in terms of the unit vectors [itex] e_\theta, e_\phi[/itex] and [itex] e_r [/itex]. Then you just substitute.