2x+y⋅3x−y⋅62x+2y = 72 2x+y⋅3x−y⋅62(x+y) = 72 2x+y⋅3x−y⋅(62)x+y = 72 2x+y⋅3x−y⋅36x+y = 72 2x+y⋅36x+y⋅3x−y = 72 (2⋅36)x+y⋅3x−y = 72 72x+y⋅3x−y = 72
At this point we can see that the equation will definitely be satisfied if
x + y = 1 and
x – y = 0
So x = 1/2 and y = 1/2 is definitely a solution.
But that doesn't necessarily mean it is the only solution.
I don't know how to prove that it is the only rational solution, but it does seem to be.
Maybe someone else can explain why that is.