The first three terms, in descending powers of \(x\), in the expansion of \(\left(3 x^{2}-a\right)^{n}\left(1+\frac{1}{x^{2}}\right)^{2}\) can be written as \(\quad 729 x^{12}+972 x^{10}+b x^{8}, \quad\) where \(a, b\) and \(n\) are constants.

Find the values of \(a, b\) and \(n\).