Solve the simultaneous equations $\displaystyle 4^x=5^y$ and $\displaystyle 2(4^x)=7^y$.

I have tried substituting $\displaystyle 5^y$ into $\displaystyle 2(4^x)=7^y$ and also changing $\displaystyle 4^x=5^y$ into $\displaystyle y=log_{5}4^x$. I seem to be going on and on without really going anywhere with my working.

Could really do with some help. Thanks