When you click on the shape below you will be able to edit the x,y coordinate pairs for the endpoints of its borders.

The x1, y1 pair describes the blue point; the x2, y2 pair describes the yellow point.

To switch the border you are editing, just click on the shape. For practice, try making the borders horizontal rather than vertical by changing the x,y numbers.

Now try changing it back to vertical, but this time drag the endpoints with the mouse. Note that the numbers change as you drag. Finally, click on "Group" and choose a fill color.