6) 3x³ - 5x²y - 9x + 15y = (3x³ - 9x) - (5x²y - 15y) = 3x(x² - 3) - 5y(x² - 3) =
= (x² - 3)(3x - 5y) = (x - √3)(x + √3)(3x - 5y)
7) m³n² - m + m²n³ - n = (m³n² + m²n³) - (m + n) = m²n²(m + n) - (m + n) =
= (m + n)(m²n² - 1) = (m + n)(mn - 1)(mn + 1)
8) ax² + ay - cy + bx² - cx² + by = (ax² + bx² - cx² ) + (ay + by - cy) =
= x²(a + b - c) + y(a + b - c) = (a + b - c)(x² + y)