$A = (2, 6, -5); B = (6, 9, 7); C = (5, 5.0); D = (3, 10, 2)$ form a parallelogram?

Question

The vertices: $A = (2, 6, -5); B = (6, 9, 7); C = (5, 5.0); D = (3, 10, 2)$ form a parallelogram?

I was able to show that the vector norm $AC=BD$ and $CB=DA$. This is enough?

Answer 1

You proved that opposite sites have the same length.
Check this against the definition of a parallelogram, and you will see that this is enough (try to construct a non-parallelogram with opposite sides having identical length - you will see you cant't).