Bonjour,
B = 3 / 4 - 4 / 7 × 21 / 10
B = 3 / 4 - 4 / 7 × 7 × 3 / 10
B = 3 / 4 - 4 × 3 / 10
B = 3 / 4 - 12 / 10
B = 0,75 - 1,2
B = - 0,45