yanaaleksandrov = 5y+21-7y=15-6y-6
5y-7y+6y=15-21-6
4y=-12
y= - 3

= 7y-12y-20=2-12y+27
7y-12y+12y=20+2+27
7y=49
y=7

1asasas Раскрываю сразу скобки
5y+21-7y=15-6y-6
-2y+21=9-6y
-2y+6y=9-21
4y=-12
y=-12/4
y=-3

7y-12y-20=2-12y-27
7y-32=-25-12y
7y+12y=-25+32
19y=7
y=7/19
y=7/19