I'm pretty new to Mathematica.
I'd like to show the intersection points in my graphic.
These are my inputs:

    f[y_] := -10 x^2 + 4000 x;
    NSolve[f[y] == 144000, x];
    Plot[{f[y], 144000}, {x, 0, 400}, PlotRange -> {0, 400000}]