Enter

y if the equation defines $y$ as a function of $x$ but not vice versa,
x if the equation defines $x$ as a function of $y$ but not vice versa,
b (for "both") if the equation defines $x$ as a function of $y$ and also $y$ as a function of $x$, and
n (for "neither") if the equation defines neither variable as a function of the other.

$y=2^x$ .