42-(34-x)=12. .........
42-(34-х)=12 34-х=42-12 34-х=30 х=34-30 х=4 проверка: 42-(34-4)=12
42-(34-x)=12 34-x=42-12 34-x=30 x=34-30 x=4 42-(34-4)=12