1) x+y=23 si z=15 xz+yz= are factor comun z
xz+yz=z(x+y)=15x23=345
2) x-y=12 si z=10 xz-yz= are factor comun z
xz-yz=z(x-y)=10x12=120
3) ab-ac=135 si a=9
ab-ac=a(b-c)
9(b-c)=135
b-c=135:9
b-c=15
4) ab+ac=165 si b+c=14
ab+ac=a(b+c)=165
ax14=165
a=165:14
a=11,785