Need Help on Assignment

Although I am Chinese,I can't understand what the question is,I am trying to translate it into Chinese.

Here's how you find one of the 3 possibilities:

The displacement vector from (1, -2, 4) to (3, 5, 7) is < 2 , 7, 3 >.
Making the same displacement from (4, 6, 8) gets you to the point (6, 13, 11), which is a valid 4th point for your parallelogram.

You find the other two possible points in a similar way.

The triangle that consists of the three points has three sides,each side can be diagonal of one parallelogram,so there are three parallelogram.As to how to find the fourth point,use the knowledge about vector.I hope you will understand it.