18-3х+12=21
-3х=21-12-18
-3х=-9
х=-9:(-3)
х=3
3(6- x)+12=21
3(6- x)=21-12
3(6- x)=9
6- x=9/3
6- x=3
х=6-3
18-3х+12=21
-3х=21-12-18
-3х=-9
х=-9:(-3)
х=3
3(6- x)+12=21
3(6- x)=21-12
3(6- x)=9
6- x=9/3
6- x=3
х=6-3
х=3