4y + 7z = 77
4y + 7z-7z = 77-7z
4y= 77-7z
4y/4= 77-7z/4
y=19.25-1.74z
4y+13z=95
4(19.25-1.74z)+13z=95
4(19.25) (4)(-1.74z)+13z=95
77-6.93z+13z=95
77+6.0z=95
77-77+6.0z=95-77
6.0z=18
6.0z/6=18/6
z=3
y=19.25-1.74z
y=19.25-1.74(3)
y=19.25-5.22=14.04