[tex](2x^9y^n)(4x^2y^{10})=8x^{11}y^{20}\\\\(2\cdot4)(x^9\cdot x^2)(y^n\cdot y^{10})=8x^{11}y^{20}\\\\8x^{9+2}y^{n+10}=8x^{11}y^{20}\\\\8x^{11}y^{n+10}=8x^{11}y^{20}\Rightarrow n+10=20\ \ \ |-10\\\\\boxed{n=10}[/tex]
[tex]\text{Used:}\\a^n\cdot a^m=a^{n+m}[/tex]
