## Intermediate Algebra for College Students (7th Edition)

$\log_b{21}$
The product rule for logarithms says that $\log_b{MN}=\log_bM+\log_bN$ i.e. the logarithm of a product is the sum of the logarithms. Hence here: $\log_b7+\log_b3=\log_b{(3\cdot7)}=\log_b{21}$