Start with
[tex]-3|2x+6|=-12[/tex]
Divide both sides by -3
[tex]|2x+6|=4[/tex]
If the absolute value of a number is 4, that number can either be 4 or -4: we have the two solutions
[tex]2x+6=4 \iff 2x=-2 \iff x=-1[/tex]
[tex]2x+6=-4 \iff 2x=-10 \iff x=-5[/tex]
