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.

$(x+1)^2+(y-4)^2 = 49$ .
$x-\sqrt{y} = 0$ .
$y=3x-6$ .
$y=\frac{1}{x-y}$ .