Bonjour,
1)
A = 7x (x + 4)
A = 7x² + 28x
B = (x - 2) (6 - x)
B = 6x - x² - 12 + 2x
B = - x² + 8x - 12
2)
B = - x² + 8x - 12
B = - 6² + 8 * 6 - 12
B = 36 + 48 - 12
B = 36 + 36
B = 72
3)
B = - x² + 8x - 12
B = - 2² + 8 * 2 - 12
B = 4 + 16 - 12
B = 20 - 12
B = 8
4) B = 0
B = (x - 2) (6 - x) = 0
(x - 2) = 0 ou 6 - x = 0
x = 2 ou x = 6