You can put this solution on YOUR website! You area expected to realize that it is a right triangle because the squares of the lengths of the sides satisfy the relationship in the Pythagorean theorem: .
So the triangle looks like this:
Since the legs of that right triangle (the sides measuring 40cm and 9cm) are perpendicular to each other, you can take one of the legs as base of the triangle, and the other as altitude (height), and calculate the area as
