Question 1012024
Recall that {{{cos(2x)=2cos^2(x)-1}}}
Set the equation equal to the one given.

{{{2cos^2(x)-1=2cos^2(y/3)-1}}}
{{{x=y/3}}}


Then substitute.
{{{2cos^2(x)-1=cos(2x)}}}
{{{2cos^2(y/3)-1=cos(2y/3)}}}