If three planes lie in a single plane, then the volume of parallelepiped will be zero. So, planes are coplanar if

The volume of parallelepiped

= (- 2. - 2 - 4.4)4 - (- 2. - 2 - 4. - 2) - 2 + (4. - 2 - (- 2). - 2) - 2

= (4 - 16)4 + (4 + 8)2 - (- 8 - 4)2

= - 48 + 24 - (- 24)

= - 48 + 48 = 0

So, planes are coplanar.