(a) The plane \(\Pi\) has equation \(x+3y+2z=1\). Find the perpendicular distance from the origin \(O\) to \(\Pi\).

(b) Relative to \(O\), points \(A,B,C\) have position vectors \(-\mathbf j+2\mathbf k\), \(2\mathbf i-\mathbf k\), and \(2\mathbf i-\mathbf j-\mathbf k\). Find the acute angle between the planes \(OAB\) and \(\Pi\).

(c) Find an equation for the common perpendicular to the lines \(OC\) and \(AB\).