Of the three coordinates, we see that one of them is at the origin. Of the remaining two coordinates, (2k,0) is on the x-axis since its y-coordinate is 0. Similarly, (0,k) is on the y-axis because it's x-coordinate is 0. Finally, we can also say that ( 2k,0) is twice as far away from the origin as (0,k).
A rectangle has four right angles which means opposite sides are parallel. This means we have to place the fourth vertex at (2k,k) to form a rectangle.
