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[tex]\boxed{ \rm{answer \: by \: { \boxed{ \rm{☆HayabusaBrainly}}}}}[/tex]
3x - 12 + 2x + 11x = 180°
16x - 12 = 180°
16x = 180 + 12
16x = 192
x = 192 ÷ 16
x = 12✔ (C.)
