$$x = a + 7 y = b - a , x + y = b + 7$$
$x=7y+a\text{ and }7y+a=b-a,x+y=b+7$
$x=8y-7$
$y=\frac{b+14}{9}\text{ and }a=\frac{b-49}{9}$
$y=a+7$
$a=\frac{b-49}{9}\text{ and }x=\frac{8b+49}{9}$
