Area Of A Parallelogram

Here is a rectangle with base b and height h, so the area [b * h]:

Now here is a parallelogram:

We can imagine this as one rectangle with two triangles:

If we shift the left triangle towards the right, we get a shape like this:

This is a rectangle, since the area of this shape didn't change, then the area of a parallelogram must also be [b * h].