Check the picture below.
[tex]\begin{cases} (12x)+(5x-10)+\theta =180\\ ~\hfill \theta +16x = 180 \end{cases} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{\textit{\Large 180}}{(12x)+(5x-10)+\theta}~~ = ~~\stackrel{\textit{\Large 180}}{\theta +16x}\implies 17x-10+\theta =\theta +16x \\\\\\ 17x-10=16x\implies 17x=16x+10\implies \boxed{x=10} \\\\[-0.35em] ~\dotfill\\\\ \theta +16x = 180\implies \theta +16(10)=180\implies \theta +160=180\implies \stackrel{\measuredangle C}{\theta} =20[/tex]
