2)6.7 ·(7,9 - y)= 28,81

3)9.14z - (3.78z + 2.87z)=12,45 

52.93-6.7y=28.81

-6.7y=28.81 -52.93 

y=24.12/6.7 

y=3.6

9.14z-3.78z-2.87z=12.45

2.49z=12.45

z=5 

6,7(7,9-y)=28,81

7,9-y=28,81:6,7

7,9-y=4,3

y=7,9-4,3

y=3,6

===============================

9,14z-(3,78z+2,87z)=12,45

9,14z-6,65z=12,45

2,49z=12,45

z=12,45:2,49

z=5