a) 3x² - 6xy + 3y² - 12z²
= 3. (x² - 2xy + y² - 4z²)
= 3. [(x² - 2xy + y²) - 4z²]
= 3. [(x - y)² - (2z)²]
= 3(x - y - 2z)(x - y + 2z)
b) 16x - 5x² - 3
= -5x² + 16x - 3
= -5x² + 15x + x - 3
= (-5x² + 15x) + (x - 3)
= -5x(x - 3) + (x - 3)
= (x - 3)(-5x + 1)