Bonsoir,
Développer :
N = (7+x)(x+3)
N = 7x+7×3+x×x+3x
N = 7x+21+x×x+3x
N = 7x+21+x²+3x
N = 10x+21+x²
↳ Loi commutative.
N = x²+10x+21
M = (5-x)(6+x)
M = 5×6+5x-6x-x×x
M = 30+5x-6x-x×x
M = 30+5x-6x-x²
M = 30-x-x²
↳ Loi commutative.
M = -x²-x+30
Bonne soirée.