(1)

(2х-5)/3=7 |×3

2х-5=21

2х=26

х=13

(2)

12/17-3а=6

6(17-3а)=12 |÷6

17-3а=2

3а=15

а=5

(3)

(5х+1)/9=9

(5х+1)/9=95х+1=81

(5х+1)/9=95х+1=815х=80

х=16

(4)

87/(6х+5)=3

3(6х+5)=87 |÷3

6х+5=29

6х=24

х=4