................................
8Si9

8. x-y=12, z=10 ⇒ xz-yz=z(x-y)=10x12= 120
9. ab+ac=420, b+c=14 ⇒ a(b+c)=420 ; inlocuim b+c=14, ax14=420 ⇒a=420:14= 30