x/7=y/3=z/1
x=7z
y=3z
8x-5y=2
8×7z-5×3z=2
56z-15z=2
41z=2
z=2/41
x=7×2/41
x=14/41
y=3×2/41
y=6/41
xyz=
=14/41×6/41×2/41=168/41