8y + z = 8

8y-8y + z = 8-8y

z = 8-8y

7y+5z=7

7y+5(8-8y)=7

7y+5(8) (5)(-8y)=7

7y+40-40y=7

40-33y=7

40-33y-40=7-40

-33y=-33

-33y/-33=-33/-33

y=1

z = 8-8(1)

z = 8-8=0