From the meaning of the question, a=C (lower 2n, upper n)=(2n! )/(n！ )(n！ ), b=C (lower 2n+ 1, upper n+ 1)=(2n+ 1! )/(n+ 1！ )(n！ )

From 13a=7b, we get 13*(2n! )/(n！ )(n！ )=7*(2n+ 1！ )/(n+ 1！ )(n！ )

Simplifying two sides, we can get13 * (n+1) = 7 * (2n+1).

So, n=6.