(2x+3y)(x-4y)
=2x²+3xy-8xy-12y²
=2x²-5xy-12y²
(5x-y)(-x-3y)
=-5x²+xy-15xy+3y²
=-5x²-14xy+3y²
(3-x)(4+y)
=12-4x+3y+xy