The formula for the area of a triangle can be derived before the formula for the area of a parallelogram is proved.

(a) Using only the formula for the area of a rectangle, find the area of a right triangle with legs a and b.

(b) Use your result from part (a) to derive the formula for the area of a triangle by using a sum or difference of the areas of right triangles.

I've got (a). I am having trouble with (b). I have no idea...