Recall that any of the four sides can be chosen as the base. You must use the altitude that goes with the base you choose.
The altitude (or height) of a parallelogram is the perpendicular distance
from the base to the opposite side (which may have to be extended). In the figure above, the altitude corresponding to the base CD is shown.
Things to try
In the figure above, click on "hide details"
Drag the orange dots on the vertices to make a random-size parallelogram.
Estimate the area of the parallelogram just counting the squares inside it
Calculate the area using the formula
When you done, click "show details" to see how close you got.