jmc

We can write $2^{x+1}=4^{x-7}$ as $2^{x+1}=2^{2(x-7)}$, which means that $x+1=2x-14$. Solving for $x$, we have $x=15$. Similarly, we can write $8^{3y}=16^{-y+13}$ as $2^{3(3y)}=2^{4(-y+13)}$, which means that $9y=-4y+52$. Solving for $y$, we have $13y=52$, so $y=4$. The value of $x+y=15+4=\boxed{19}$.