3х - 6 =4х
6х-21=9
3х-4+2х=6+2х
5х+15=10
4-2у=у-5
50-7у-16=3у-18

3х - 6 =4х  

4x - 3x = -6

x = -6

6х-21=9

6x = 9 + 21

6x = 30

x = 30 : 6

x = 5

3х-4+2х=6+2х  

5x - 4 = 6 + 2x

5x - 2x = 6 + 4

3x = 10

x = 10 : 3

x = 3 1/3

5х+15=10

5x = 10 - 15

5x = -5

x = -5 : 5

x = -1

4-2у=у-5

y + 2y = 5 + 4

3y = 9

y = 9 : 3

y = 3

50-7у-16=3у-18

34 - 7y = 3y - 18

3y + 7y = 34 + 18

10y = 52

y = 52 : 10

y = 5,2