ok guys here’s what I find from my teacher.
3xy-y=21
y(3x-1)=21 from here it results that y divides 21,so you obtain this possible solution for y={1,-1,3,-3,7,-7,21,-21}
And now you try every possible y on the equation and you will find that the solutions are:
(y,x)={(-3,-2);(-21,0)}