My curve equation is: y = -0.0009x^2 + 1.9113x + 4.2096

As an example, my first y = 765, but I can't for the life of me work out the x value.

I know the quadratic equation: x=(-b+√b2-4ac)/2a, but I don't know where the y value comes into it, as this equation only solves for where y = 0.

Any help would be greatly appreciated!