[tex]\it 2a-b=7\ \ \ \ \ (1)
\\\;\\ \\\;\\ \dfrac{^{2)}a}{\ \ 5} = \dfrac{b}{3} \Rightarrow \dfrac{2a}{10} = \dfrac{b}{3} = \dfrac{2a-b}{10-3} \stackrel{(1)}{=} \dfrac{7}{7} =1 \ \ \ \ (2)
\\\;\\ \\\;\\
(2) \Rightarrow \dfrac{2a}{10} = 1 \Rightarrow 2a = 10 \Rightarrow a = 5[/tex]