13.04 Vectors in 3D

State the set of points $\left(x,y,z\right)$(x,y,z) defined by the equation $z=5$z=5.

Find the equation of a sphere with centre $\left(5,2,-6\right)$(5,2,−6) and radius $2$2 units.

Find the distance from $\left(0,0,0\right)$(0,0,0) and $\left(2,5,1\right)$(2,5,1).