a)
[tex]( - x + y)(5x + 6y) = - 5 {x}^{2} - 6xy + 5xy + 6 {y}^{2} = - 5 {x}^{2} - xy + 6 {y}^{2} [/tex]
b)
[tex](x + 5y)(7x - y) = 7 {x}^{2} - xy + 35xy - 5 {y}^{2} = {x}^{2} + 34xy - 5 {y}^{2} [/tex]
c)
[tex](2x - y)(x + y) = 2 {x}^{2} + 2xy - xy - {y}^{2} = 2 {x}^{2} + xy - {y}^{2} [/tex]
d)
[tex](ab - a)(2ab + 6a) = 2 {a}^{2} {b}^{2} + 6 {a}^{2} b - 2{a}^{2} b - 6 {a}^{2} = 2 {a}^{2} {b}^{2} + 4 {a}^{2} b - 6 {a}^{2} [/tex]
