Show that the four points A, B, C and D with position vectors
 respectively are coplanar.

Given position vectors of four points A, B, C and D are:


These points are coplanar, if the vectors,  are coplanar. 



These vectors are coplanar if and only if, they can be expressed as a linear combination of other two. 
So Let


Comparing the coefficients, we have, 


These values of x and y satisfy the equation  3x + 3y = -2.
Hence the vectors are coplanar.