Using the addition principle which states that:
[tex]\text{If a = b, then a+c=b+c}[/tex]In our case, we add 9 to both sides to get:
[tex]\begin{gathered} -9+x=-12 \\ -9+x+9=-12+9 \\ x=-3 \end{gathered}[/tex]Performing our check,
[tex]\begin{gathered} -9+x=-12 \\ -9+(-3)=-9-3=-12 \end{gathered}[/tex]Therefore, x = -3
