Exercice 29 et 30 svp :) merci

Exo29:
D=(x-7)(5x+11)
D=5x²+11x-35x-77
D=5x²-24x-77

Si x=7
D=(7-7)(5*7+11)=0

Exo31:
F=4(7-5a)+5(2a-3)
F=28-20a+10a-15
F=-10a+13

si a=3/10
F=-10*3/10+13=-3+13=10