## Algebra 1: Common Core (15th Edition)

$1/b^{21}$.
When a power is raised to a power, we multiply the two powers. Thus, we multiply -7 and 3 to obtain that the expression simplifies to $b^{-21}$. However, because -21 is negative, we put it in the denominator of the fraction, obtaining that this simplifies to: $1/b^{21}$.