a)
(3x - 9y) + (5y + 6x) = 3x - 4y + 6x = 9x - 4y
b)
(4a + b) - (-4a + 5b) = 4a + b - (-4a) - 5b = 4a + b + 4a - 5b = 8a + b - 5b = 8a - 4b
c)
(2x-3y)(5y-3x) = 10xy - [tex]6x^{2}[/tex] - [tex]15y^{2}[/tex] + 9xy = 19xy - [tex]6x^{2}[/tex] - [tex]15y^{2}[/tex]
