$(4$ marks$)$ Given ${\displaystyle \underset{1}{\overset{2}{}}}(f(x)+g(x))dx={\displaystyle \underset{2}{\overset{3}{}}}(2f(x)-g(x))dx$ and ${\displaystyle \underset{1}{\overset{3}{}}}g(x)dx=3{\displaystyle \underset{2}{\overset{3}{}}}f(x)dx+1$

Find ${\displaystyle \underset{1}{\overset{3}{}}}f(x)dx$