Binomial Theorem

If the coefficient of \({x^7}\) in \({\left( {ax - {1 \over {b{x^2}}}} \right)^{13}}\) and the coefficient of \({x^{ - 5}}\) in \({\left( {ax + {1 \over {b{x^2}}}} \right)^{13}}\) are equal, then \({a^4}{b^4}\) is equal to :