Vertex representation of basketball throwing

basketball throwing has approximately a shape of parabola. Hence, we can find variables a,p & k such that f(x)=a(x-p)^2+k will represent a parabola that the ball's orbit pass through it. In this applet you can try to find variables that will match to the ball's orbit in the picture.
Have you manage to find a matching variables? what are they? can you explain in your own word what each of the variables act for?