My teacher taught the class how to do problems dealing with coplanar vectors with three points and now the book is asking for the reader to solve with four points. In the book there is no explanation on how to solve it. The question is found below:

5.Determine if the points are coplanar.

a)

b)

c)

The book says a and c are not coplanar and b is coplanar. How does one solve if it is coplanar?