#### Answer

$a=4$

#### Work Step by Step

We start with the given equation: $-3a+21=9$
We subtract $21$ from both sides of the equation: $-3a+21-21=9-21$
We combine like terms on both sides of the equation: $-3a=-12$
We divide by $-3$ on both sides of the equation: $a=\frac{-12}{-3}=4$