If the coefficient of 3rd term from the beginning in the expansion of $\left(a x^2-\frac{8}{b x}\right)^9$ is equal to the coefficient of 3rd term from the end in the expansion of $\left(a x-\frac{2}{b x^2}\right)^9$, then the relation between $a$ and $b$ is

If $X \sim B(7, P)$ is a binomial variate and $P(X=3)=P(X=5)$, then $P=$