Question 468345
From the first equation {{{2^x=8^(y+1)}}} 
{{{2^x=2^(3(y+1))}}} 
So, {{{x=3(y+1)}}}
From the second equation {{{9^y=3^(x-9)}}}
{{{3^(2y)=3^(x-9)}}}
{{{2y=x-9}}}
System
{{{system(x=3(y+1),2y=x-9)}}}
{{{system(x-3y=3,x-2y=9)}}}, 
Subtract the second equation from the first one
{{{-y=-6}}}
{{{y=6}}}
Put {{{y=6}}} in the first equation
{{{x-3*6=3}}}
{{{x=21}}}
{{{system(x=21,y=6)}}} 
{{{x+y=21+6=27}}}
Answer: 27