[tex]\displaystyle 7a+3b=85 \\ a=2b \\ 7a+3b=85 \Rightarrow 7 \cdot 2b+3b=85 \Rightarrow 14b+3b=85 \Rightarrow 17b=85 \Rightarrow \\ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ \Rightarrow b= \frac{85}{17} \Rightarrow \boxed{b=5} \\ a=2b \Rightarrow a=2 \cdot 5 \Rightarrow \boxed{a=10}[/tex]
