bearing in mind that an absolute value expression is in effect a piece-wise expression, because it has a ± version.
[tex] \bf 3|x|+7=28\implies 3|x|=21\implies |x|=\cfrac{21}{3}\implies |x|=7\implies
\begin{cases}
+(x)=7\\
-(x)=7
\end{cases}
\\\\[-0.35em]
~\dotfill\\\\
+(x)=7\implies \boxed{x=7}~\hfill -x=7\implies \boxed{x=-7} [/tex]
