This works through area of a parallelogram and then uses this to find the area of a trapezium.
I normally use the parallelogram animation a few times as it always seems to get a cheer.
For the trapezium, students cut out their own trapezium from one piece of paper and then make an identical one from the other piece they have. By rotating this second and putting it next to the first they should have a parallelogram. You can run through this on the board using your own trapezia and then summarise the formula with the class.