I have read, from the question:

Irreducibility of polynomials in two variables, that all polynomials $f(x)-g(y)$, where $f, g$ are indecomposable polynomials, and there are no $a, b$ such that $g(ax+b)=f(x)$, are irreducible, unless the degrees of $f$ and $g$ are $$7, 11, 13, 15, 21,\ \ \text{or} \ \ 31$$

Is there an example of the exceptional case in degree $7$, with the factorization?