How is finding the area of a parallelogram like finding the area of two congruent triangles?

If you look at either diagonal (line joining opposite angles) it forms 2 triangles that are identical, both in size and shape, but that are in different positions. So by finding the area of one using base x height ÷ 2, and then adding them together, you get the area of the polygon.
However, you will find that that would be the same as just doing base x height, so the area of a parallelogram is just that.

The correct answer is -4