4 The points $A , B , C$ have position vectors

$$- \mathbf { i } + \mathbf { j } + 2 \mathbf { k } , \quad - 2 \mathbf { i } - \mathbf { j } , \quad 2 \mathbf { i } + 2 \mathbf { k } ,$$

respectively, relative to the origin $O$.\\
(a) Find the equation of the plane $A B C$, giving your answer in the form $a x + b y + c z = d$.\\

(b) Find the perpendicular distance from $O$ to the plane $A B C$.\\

(c) Find the acute angle between the planes $O A B$ and $A B C$.\\

\hfill \mbox{\textit{CAIE Further Paper 1 2020 Q4}}