A² - 4ab + 3b² = a² - (ab + 3ab) + 3b² = (a² - ab) - (3ab - 3b²) =
= a(a - b) - 3b(a - b) = (a - b)(a - 3b)
x² + 7xy - 8y² = x² + (8xy-xy) - 8y² = (x² - xy) + (8xy - 8y²) =
= x(x-y) + 8y(x-y) = (x-y)(x+8y)
2x² - 5xy + 3y² = 2x² - (2xy + 3xy) + 3 y² = 2x² - 2xy - 3xy + 3 y² =
= (2x² - 2xy) - (3xy - 3y²) = 2x(x - y) - 3y(x - y) =
= (x - y)(2x - 3y)