ABCD is a parallelogram. The position vectors of A,B and C are given by A= $\ 2i+j+3k $ ,$\ B =3i-2j $ , $\ C= i-j-2k $, respectively. Find the position vector of D and calculate the angle ABC.
My working vector of $\ D = A+B+C $ = .............
I know how to calculate 2d vector angle but i do not know how to do in 3d vectors