Chapter 2 - Solving Equations - 2-3 Solving Multi-Step Equations - Mixed Review - Page 100: 74

$a=4$

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: $a=\frac{-12}{-3}=4$