This self checking worksheet works on the multiplication step after 'FOIL:' (4x-3y)(5x^2-3xy+y^2). The worksheet has very similar binomials and trinomials to multiply together making the solution hard to guess. This worksheet is certain to stump the sharpest student in your class if they are not careful. Keeping track of the positive and negatives signs are crucial to finding the correct solution. Upon completion of this worksheet, the students will discover the solution to a riddle.