Answer
$$\left( {0,0, - 6} \right)$$
Work Step by Step
$$\eqalign{
& \left( {6,\pi ,\frac{\pi }{2}} \right) \cr
& \left( {\rho ,\theta ,\phi } \right):{\text{ }}\left( {6,\pi ,\frac{\pi }{2}} \right) \to \rho = 6,{\text{ }}\theta = \pi ,{\text{ }}\phi = \frac{\pi }{2} \cr
& {\text{Spherical to rectangular }}\left( {\rho ,\theta ,\phi } \right) \cr
& x = \rho \sin \phi \cos \theta ,{\text{ }}y = \rho \sin \phi \sin \theta ,{\text{ }}z = \rho \cos \phi \cr
& x = \left( 6 \right)\sin \left( \pi \right)\cos \left( {\frac{\pi }{2}} \right) = 0 \cr
& y = \left( 6 \right)\sin \left( \pi \right)\sin \left( {\frac{\pi }{2}} \right) = 0 \cr
& z = \left( 6 \right)\cos \left( \pi \right) = - 6 \cr
& {\text{The rectangular }}\left( {x,y,z} \right){\text{ coordinates are:}} \cr
& \left( {0,0, - 6} \right) \cr} $$
