Supplement D9.) On a regular geoboard it took a lot of rubberband maneuvering to figure out how to change a rectangle into a non-rectangular parallelogram and keep the same area. We decided to use the (free) Geoboard App to make it easier. (Available in both online and app versions.)
To the above right, you can see the original rectangle. Below you'll see the rubberband movement that resulted in a non-rectangular parallelogram. I took a photo and quickly scribbled down how my student calculated the total area. You'll notice that he combined the sides to calculate the area.
In my more recent math education (teaching!) it's been enlightening for me to use manipulatives like this to calculate area of shapes.
Who knew it was so easy to calculate area? :)