What is the difference if a quadratic equation is set equal to 0 or to y?
For example:
y = x^2 - 8x + 15
OR
x^2 - 6x - 16 = 0
y = x^2 - 8x + 15
the equation becomes a function and its graph is a parabola.
if you have x^2 - 8x + 15 = 0 you are looking for the x-intercepts of that parabola.
You would also be looking for those values of x which makes the equation true, also called the roots of the equation.