1/3х + 1/7у - 1/7х +1/2у =
= 1/3х - 1/7х +1/7у + 1/2у =
= 7/21х - 3/21х 2/14у + 7/14 у =
= 4/21х + 9/17у
1/3х + 1/7у - 1/7х +1/2у =
= 1/3х - 1/7х +1/7у + 1/2у =
= 7/21х - 3/21х 2/14у + 7/14 у =
= 4/21х + 9/17у