jekadva 8у-3(2у-3)=7у-2(5у+8);
8y-6y+9=7y-10y-16
2y+9=-3y-16
2y+3y=-16-9
5y=-25
y=-25:5
y=-5

5(2у-9)+6ц=4(3у-2)-21;
10y-45+6=12y-8-21
10y-39=12y-29
10y-12y=-29+39
-2y=10
y=-10:2
y=-5

5у+7(3-у)=3(5-2у)-6; 
5y+21-7y=15-6y-6
-2y+21=9-6y
-2y+6y=9-21
4y=-12
y=-12:4
y=-3

7у-4(3у+5)=2-3(4у-9)
7y-12y-20=2-12y+27
-5y-20=-12y+29
-5y+12y=29+20
7y=49
y=49:7
y=7