Can the quadratic formula be used with variable coefficients? Could we use the quadratic formula on an expression such as $z^2xy - zx^2y+y = 0$ to find $z$ in terms of $x$ and $y$?
 A: Write $$z^2xy - zx^2y + y = 0$$ 
as $$(xy)z^2 - (x^2y)z + (y) = 0$$ 
so that you can see $a = xy$, $b=-x^2y$ and $c=y$.
Then 
\begin{align}
   z  &= \dfrac{-b \pm \sqrt{b^2-4ac}}{2a} \\
   z  &= \dfrac{x^2y \pm \sqrt{x^4y^2-4xy^2}}{2xy} \\
   z  &= \dfrac{x^2y \pm |y|\sqrt{x^4-4x}}{2xy} \\
   z  &= \dfrac{x^2 \pm \sqrt{x^4-4x}}{2x} \\
\end{align}
Notice that $y$ has disappeared. That is because 
$(xy)z^2 - (x^2y)z + (y) = 0 \iff y(xz^2 - x^2z + 1) = 0$.
So we need to notice that $y=0$ is a solution. If $y\ne 0$ then we get
$z = \dfrac{x^2 \pm \sqrt{x^4-4x}}{2x}$; where we are going to need to require that $x \ne 0$ and, if we only want real-valued solutions, $x^4-4x \ge 0$.
A: Let us illustrate the usefulness of this technique for "splitting" an implicit equation into a set of cartesian equations. This will, at the same time, show the significance of condition $\Delta>0.$
Consider implicit degree 2 equation: $$x^2+xy+y^2=1.$$ 
If we want to draw its curve (a conic section), we can consider its equation as a quadratic equation with variable $y$ and parameter $x$:
$$y^2 + xy + (x^2-1)=0$$
Its discriminant is : $\Delta=x^2-4 (x^2-1)=4-3x^2$
There, a discussion has to take place:
Under the condition that $\Delta=4-3x^2 \geq 0$, i.e., $x \in I:=[-\dfrac{2}{\sqrt{3}},\dfrac{2}{\sqrt{3}}]$, we have two solutions:
$$\begin{cases}y_1:=f_1(x)=\frac12(-x-\sqrt{4-3x^2}) \ \  \text{(blue curve)}\\y_2:=f_2(x)=\frac12(-x+\sqrt{4-3x^2}) \ \  \text{(red curve)}\end{cases}$$
corresponding to the cartesian equations of the lower and upper curves that can be seen on the figure. The union of these curves is an ellipse whose projection onto the $x$ axis is interval $I$.

A: Yes, jolly well... Your surface has equation
$$ y(z^2x-zx^2+1)=0 $$
in which $y=0 $ is seen as a factor representing the $(x,\, z)$ plane. Leave it out temporarily while trying to find $z$ as a function of $x,y$.
You can now for example recognize $$  (2x-y+1)( x+y-2)=0 $$ as two straight lines in 2D or corresponding extruded planes in 3D.
Similarly we have two cylinders of given profiles prismatically extruded.
You find in the remaining quadratic $ (z^2x-zx^2+1)=0 $  factorizations with two factors is  possible.. where the roots are:
$$ 2z = x\pm \sqrt{x^2-5/x} $$
where we treated $x$ as constants while handling it as a quadratic.

Each of the factors (constituents)  represents a cubic hyperbolic cylinder (green,brown) whose product surface is also represented  in the graphic. The $ (x,z)$ plane is omitted in visualization.
A: Yes!  A variable just stands for a number we don't know, so anything you can do with numbers you can also do with variables.  (Actually, sometimes a variable stands for a number we do know, but either way, it still is just a number.)
There is one thing to be careful of, though.  The quadratic formula is only valid when you have a genuine quadratic equation, which means the coefficient of $z^2$ must be nonzero.  So in this case, you can only use the quadratic formula to solve for $z$ in terms of $x$ and $y$ assuming that $xy\neq 0$ (or equivalently, that both $x\neq 0$ and $y\neq 0$).
A: Yes! In fact, you probably learned it with variable coefficients; e.g. in the form of solving the equation
$$ a x^2 + bx + c = 0 $$
when given the presumption that $a \neq 0$.
