a) x2 - xy - 20y2
= x2 + 4xy - 5xy - 20y2
= x( x + 4y ) - 5y( x + 4y )
= ( x + 4y )( x - 5y )
b) x3 - x2y - 3xy2 + 2y3
= x3 + x2y - 2x2y - xy2 - 2xy2 + 2y3
= ( x3 + x2y - xy2 ) - ( 2x2y + 2xy2 - 2y3 )
= x( x2 + xy - y2 ) - 2y( x2 + xy - y2 )
= ( x2 + xy - y2 )( x - 2y )