I haven't done any vector geometry in close to 2 years now and I got this question for one of my exercises and I've completely forgotten what to do.

Question: Show that the points A(1, 0, 1), B(2,−1, 3), C(5, 1,−1) and D(6, 0, 1) are vertices of a parallelogram. Is the parallelogram a square?

A full solution along with a step by step explanation would be great.. Just can't seem to remember how to do questions like this.

Thx in advance.