Answer
$-4x^{11}y^{12}$
Work Step by Step
Based on the product rule for exponents, we know that $a^{m}\times a^{n}=a^{m+n}$ (where $m$ and $n$ are positive integers and $a$ is a real number).
Therefore, $(-x^{9}y)(4x^{2}y^{11})=-1\times4\times x^{9+2}\times y^{1+11}=-4\times x^{11}\times y^{12}=-4x^{11}y^{12}$.