Respuesta :

TSO
[tex]a = \frac{1}{3} (a+b+c)\\\\3a = a + b + c\\\\ 3a - a = a - a + b + c\\\\2a = b + c\\\\2a - b = b - b + c\\\\2a-b = c\\\\\\ \boxed{\bf{c = 2a - b}}[/tex]