7y + 6z = 177 y + z = 28

y + z = 28

y+z-z=28-z

y=28-z

7y + 6z = 177

7(28-z)+6z=177

196-7z+6z=177

196-z=177

196-196-z=177-196

-z=-19

-1z/-1=-19/-1

z=19

y=28-z

y=28-19

y=9