The area of a parallelogram or rectangle is base length times height. But in the case of the rectangle, its height is its width. In the case of the parallelogram the height needs to be calculated knowing the slant length and the angle the slant side makes with the base. If this angle is θ and the slant length is s, then the height is ssinθ. If the base length is b the area is sbsinθ. If the width of the rectangle is s, then its area is just sb. (In other words θ=90º, and sin90=1.)