Solve for: (-3a)/(-21b)

Expression: $\frac{-3a}{-21b}$

Apply the fraction rule $\frac{-a}{-b}=\frac{a}{b}$

$=\frac{3a}{21b}$

Factor the number: $ 21=3\cdot 7$

$=\frac{3a}{3\cdot 7b}$

Cancel the common factor: $ 3$

$=\frac{a}{7b}$