a) (x + 1)(2x - 4) = x*2x - 4x + 2x - 4 = 2x² - 4x + 2x - 4 = 2 x² - 2x - 4
b) (x + 7)(2x - 3) = x*2x - 3x + 7*2x - 7*3 = 2x² - 3x + 14x - 21 = 2x² + 11x - 21
c) ( -x + y)( 10x + 3y) = -x*10x - 3xy + 10xy + y*3y = -10x² - 3xy + 10xy + 3y² =
= -10x² + 7xy + 3y²