Bonjour,
(a - b)² = a² - 2ab + b²
(12x + 3)(2x - 7) - (2x - 7)²
= 12x * 2x + 12x * (-7) + 3 * 2x + 3 * (-7) - ((2x)² - 2 * 2x * 7 + 7²
= 24x² - 84x + 6x - 21 - (4x² - 28x + 49)
= 24x² - 78x - 21 - 4x² + 28x - 49
= 24x² - 4x² - 78x + 28x - 21 - 49
= 20x² - 50x - 70
