Chapter 3 - Equations and Problem Solving - 3.2 - Equations and Problem Solving - Problem Set 3.2 - Page 106: 51

The number is $9$

$\text{Let}$ $x$ $\text{represent the certain number.}$ Since $12$ is added to this number, we write $12+x$. Since that sum "is" $21$, we write that it is equal to $21$. When we combine these two statements, we get the equation $12+x=21$. To solve this equation for $x$, we must subtract $12$ from both sides: $$12-12+x=21-12$$$$x=9$$