The three parallelograms below all have two sides that are 3 units long and two sides that are 7 units long.[br] [br](a) Use the moving and additivity principles to determine the areas of the three parallelograms.[br](b) Can there be a formula for areas of parallelograms that is only in terms of the lengths of the sides?

Find a formula for the area of a parallelogram [i]in terms of lengths of parts of the parallelogram[/i].