This isn't as difficult as it seems
"D" can be figured as one of three possible points
First possibility...... add A and B and subtract C....so we have
(-2 + 4 - 2, 3 + 2 - -1) = ( 0, 6) = (x,y) = D1
Second possibility....add B and C and subtract A.....so we have
(4 + 2 - -2 , 2 - 1 - 3) = ( 8, -2) = (x,y) = D2
Third possibility.....[ you might have already guessed it !! ]....add A and C and subtract B
(-2 + 2 - 4, 3 - 1 - 2) = ( -4, 0) = (x,y) = D3
If the coordinates have to follow a clock-wise order, then D3 will be the "correct" point for "D"
Here's a graph showing the possible parallelograms :

