O ângulo externo de um triângulo é igual a soma dos ângulos internos não adjacentes a ele, ou seja :
[tex]3\text x-30 =\text x-10+ \text x+30[/tex]
[tex]3\text x-30 =2\text x+20[/tex]
[tex]\boxed{\text x = 50}[/tex]
Ângulos internos :
[tex]\text A = \text x-10 \to \text A = 50-10\\\\ \to \huge\boxed{\text A=40}[/tex]
[tex]\text B = \text x+30 \to \text B = 50+30\\\\ \to \huge\boxed{\text B=80}[/tex]
[tex]\text{A+B+C = 180} \to 40+80+\text C =180 \\\\\to \text C =180-80-40 \\\\\to \huge\boxed{\text C = 60}[/tex]
