Answer
$y^{9b}$
Work Step by Step
Based on the quotient rule for exponents, we know that $\frac{a^{m}}{{a}^{n}}=a^{m-n}, a\ne0$ (where $m$ and $n$ are positive integers and $a$ is a real number).
Therefore, $\frac{y^{15b}}{y^{6b}}=y^{15b-6b}=y^{9b}$.