1. 18+х=18
х=18-18
х=0
2. у-5=38-5
у=38-5+5
у=38
3. 80-z=80-0
z=80-0-80
z=0
4. 76-а=20+76
а=20+76-76
а=20
Вроде так
<span> \left \{ {{2y-4>3y+1} \atop {5y+5<4y+3}} \right. \left \{ {{-y\ \textgreater \ 5} \atop {y\ \textless \ -2}} \right. \left \{ {{y\ \textless \ -5} \atop {y\ \textless \ -2}} \right. y\ \textless \ -5</span>
1)35*7=245
2)199*3=597
3)245-597= -352