Rewrite power series $x{\displaystyle \underset{n=1}{\overset{}{}}}(n9)x^{n-1}{\displaystyle \underset{n=1}{\overset{}{}}}(n7)x^{n-1}$ as a single series in terms of $x^n.$

$x{\displaystyle \underset{n=1}{\overset{}{}}}(n9)x^{n-1}{\displaystyle \underset{n=1}{\overset{}{}}}(n7)x^{n-1}=A{\displaystyle \underset{n=1}{\overset{}{}}}B{x}^n$

where $A=$

and

the expression $B=$