Answer:
[tex]\large\boxed{\left \{ {{x\: = \: 8} \atop {x\: =\: -6}} \right. }\\[/tex]
Step-by-step explanation:
[tex]\left| -3x+3 \right| -9=12[/tex]
At first, let's add - 9 to both the sides. So,
[tex]\left| -3x+3 \right| -9=12\\|-3x+3|=21[/tex]
Now, using the definition of absolute value, we can change the equation to..
[tex]-3x+3=21 \\-3x+3=-21[/tex]
By, subtracting 3 from both the sides,
[tex]-3x=18 \\-3x=-24[/tex]
Now, divide both the sides by 6,
[tex]x=-6 \\x=8[/tex]
We now have 2 values of x. They are...
[tex]\large\boxed{\left \{ {{x\: = \: 8} \atop {x\: =\: -6}} \right. }\\[/tex]
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Hope it helps!
[tex]\mathfrak{Lucazz}[/tex]