Hi there!
The problem statement describes the dimensions. The base length is 15 ft; the height is 10 ft.
The area of a parallelogram is the product of base length and height, just as it is for a rectangle.
A = bh
A = (15 ft)(10 ft) = 150 ft²
Mr Morris should purchase 150 ft² of material.
_____
If you compare the area to a 15 ft by 10 ft rectangle, you see the "excess" triangle at the left end just matches the "missing" triangle at the right end. The A=bh formula still works even if the top and base are offset by more than the width of the base.
~SharkieOwO~