3-D geometry : three vertices of a ||gm ABCD is (3,-1,2), (1,2,-4) & (-1,1,2). Find the coordinate of the fourth vertex. The question is

Three vertices of a parallelogram ABCD are A(3,-1,2), B(1,2,-4) and C(-1,1,2). Find the coordinate of the fourth vertex.

To get the answer I tried the distance formula, equated AB=CD and AC=BD.

use mid point formulae
let point be $d(a,b,c)$
so,
     $$\frac{(a+1)}2 = \frac{(3-1)}2$$
so $a=1$
similarly find c ad b as well
