Use the Gram-Schmidt process to construct an orthonormal basis for the following subspaces of $\mathbb{R}^3$ : (a) the plane spanned by $(0,2,1)^T,(1,-2,-1)^T ;$ (b) the plane defined by the equation $2 x-y+3 z=0 ;(c)$ the set of all vectors orthogonal to $(1,-1,-2)^T$.