7y^2 = 9 - 15x^2

The right hand side is divisible by 3, so y must be divisible by 3.

But if y is divisible by three, then 9 - 7y^2 is divisible by 9, so x must be divisible by 3.

But xdog has already proved that x and y cannot both be divisible by 3, so we are done.