Bonjour,
6x + 9 = -3
6x = -3 - 9
6x = -12
x = -12/6
x = -2
6x - 31 = 4x + 9
6x - 4x = 31 + 9
2x = 40
x = 40/2
x = 20
A = 4(5n - 9)
A = 4 * 5n + 4 * (-9)
A = 20n - 36
B = (7x + 5)(3x - 8)
B = 7x * 3x + 7x * (-8) + 5 * 3x + 5 * (-8)
B = 21x^2 - 56x + 15x - 40
B = 21x^2 - 41x - 40