ex 8 pls pls pls plsp

ZA+ZB+ZC=20=>ZA+ZB=20-ZC|
ZA-ZB=ZC+2 |=>ZC+4=20-ZC
ZA+ZB-ZC=4=>ZA+ZB=ZC+4 |2ZC=16
ZC=8
ZA+ZB=14|
|=>ZA=2
ZA-ZB =10| ZB=12