Sorry  this is not an InterActivity yet and shouldn't be here ... but it is coming.
Interactivity  Construct a parallelogram ABCD with vertices A, B and C
Directions for interactivity
In the top right: Reset button to start over or Undo button to undo last step.
First draw sides \overline{AB} and \overline{BC} How?
 Select the line segment tool and click on A and then on B (they will glow).
 You will get a line segment a joining A and B.
 Click on B and then on C.
 You will get a line segment b joining B and C.
Now pick any of the 3 ways below to draw a parallelogram. You will always get the point D(3,4)!
One way to draw the parallelogram (based on definition that opposite sides are parallel). How?
 Select the parallel line tool and click on point A and then on line b.
 You will get a line c passing through A and parallel to b=\overline{BC} .
 Click on point C and then on line a.
 You will get a line d passing through C and parallel to a=\overline{AB} .
 Click on the intersection tool and then click on the intersection point of c and d.
 You will get the point D at (3,4).
Another way to draw the parallelogram (based on definition that opposite sides are the same length). How?
*Select the compass tool and click on center point A and then on B and on C.
 You will get a circle c with center A and radius the length of b=\overline{BC} .
 Click on point C and then on A and on B
 You will get a circle d with center C and radius the length of a=\overline{AB} .
 Click on the intersection tool and then click on the intersection point of c and d.
 You will get the point D at (3,4).
A third way to draw the parallelogram (based on definition that the diagonals bisect each other). How?
 (With the line segment tool still selected, draw the diagonal c=\overline{AC} .
 Select the midpoint tool and click on c.
 You will get the midpoint D of b=\overline{AC} . Rightclick on the label "D", select rename and type in E and click on OK.
 Click on the line tool and click on B and then on E to get the line d.
 Click on the circlecenterpoint tool and then click on E and then on B.
 You will get a circle e with center E and radius the length of a=\overline{EB} . (It will pass through B.)
 Click on the intersection tool and then click on the intersection point of line d and circle e.
 You will get the point D at (3,4).

How to save interactivity
 To save and then use the saved file, you will need to download and install GeoGebra
 If there is a menubar, click on File > Save.
 If there is no menubar, doubleclick anywhere in the interactivity. A separate GeoGebra window will open (with menubar).
Click on File > Save.
