Решение 

 

restart; 1; solve({`+`(y, `-`(x)) = 2, `+`(`/`(`*`(840), `*`(x)), `-`(`/`(`*`(780), `*`(y)))) = 4}) 

{x = -15, y = -13}, {x = 28, y = 30} (16.2.1)