So I have a parabolic equation in the form of Y=ax^2+bx+c. How do I get the one that solves distance?

So I have and equation where given a distance of my projectile launcher, I can determine a neccessary target height, and vice versa. But how do I figure out (given an x coordinate) how far away my projectile launcher goes? My math teacher said something about a bunch of Xs but I lost it. Thanks!