This problem deals with the question of whether an equation defines one of the variables as a function of the other. You can answer it by seeing if the equation can be solved for one variable in terms of the other.
For example, the equation $$y-x^2=0$$ can be rewritten as $$y = x^2$$ and so any choice of $x$ uniquely determines $y$. We can think of $y$ as being given by the function $$y = f(x) = x^2.$$ On the other hand, for a particular value of $y$, e.g., $y=1$, there are two values of $x$, i.e., $x=\pm 1$, and so the equation $y-x^2=0$ does not define $x$ as a function of $y$. Similarly, by switching the roles of $x$ and $y$ we see that the equation $$y^2-x=0$$ defines $x$ as a function of $y$, but not $y$ as a function of $x$.
By contrast, the equation $$3x+4y=5$$ determines $x$ as a function of $y$ and also determines $y$ as a function of $x$, while the equation $$x^2+y^2 = 1$$ defines neither variable as a function of the other.
For the following equations, 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.

These equations are the same as discussed above, so you can check that you understand how to answer this and the following questions:

$y-x^2 = 0$ .
$y^2-x = 0$ .
$3x+4y+5=0$ .
$x^2+y^2=1$ .