Now the sophisticated way to do this would be using linear algebra. But i'm too lazy, so i'm going to suggest something more elementary, that to me makes sense would work, and if anyone finds a problem with it, they can point it out.
you want to reflect in the line y = x - 1. (This means also that x = y + 1), so make these substitutions:
The curve becomes
Now, solve for . And note that this is not a function. You can graph it piece-wise though