You can do this without drawing even. Let's say the coordinates are A, B and C, respectively, and we're looking for D.
If the other vertex is opposite A(2,1), it will be at B+C-A = (8, 11) If the other vertex is opposite B, it will be at A+C-B = (4, -1) If the other vertex is opposite C, it will be at A+B-C = (0, 3)
Explanation: think of one of the vertexes as a corner of the parallelogram and you want to travel to the opposite corner. That means you must add both vectors that represent the sides. So from A to D, you'd have to add (B-A) and (C-A). That gives us A+(B-A)+(C-A) simplifies to B+C-A.
