Check the picture below.
[tex]8x-16=4x+20\implies 4x-16=20\implies 4x=36\implies x=\cfrac{36}{4}\implies \boxed{x=8} \\\\\\ (8x-16)~~ + ~~(4x+20)\implies 12x+4\implies 12(8)+4\implies \LARGE \begin{array}{llll} \stackrel{\measuredangle ABC}{100^o} \end{array}[/tex]
