a)(2x-7y)(4x+3y) б)(5x-3y)^2

(2x-7y)(4x+3y)=-21*y^2-22*x*y+8*x^2
(5x-3y)^2=9*y^2-30*x*y+25*x^2

a)(2x-7y)(4x+3y) = 2x * (4x + 3y) - 7y * (4x + 3y) = 8x^2 + 6xy - 28xy - 21y^2 = 8x^2 - 22xy - 21y^2
б)(5x-3y)^2 = (5x)^2 - 2*5x*3y + (3y)^2 = 25x^2 - 30xy + 9y^2
или (5x-3y)^2 = (5x - 3y)*(5x - 3y) = 5x*(5x - 3y) - 3y*(5x-3y) = 25x^2 - 15xy - 15xy + 9y^2 = 25x^2 - 30xy + 9y^2