Check the picture below.
[tex]a+b=180\implies b=180-a \\\\\\ \stackrel{\textit{substituting on the 2nd equation}}{a-39=2(180-a)}\implies a-39=360-2a\implies 3a-39=360 \\\\\\ 3a=321\implies a = \cfrac{321}{3}\implies \boxed{a = 107}~\hfill \boxed{\stackrel{180-107}{b = 73}}[/tex]
