The constant <em>a</em> is the scale factor on the parabola; positive <em>a</em> makes a CUP (concave up), which is the usual orientation. Negative <em>a</em> makes an upside-down parabola. Big <em>a</em> (in absolute value) makes a skinny parabola, small <em>a</em> a wider one.
<em>r</em> and <em>s</em> are the zeros of the quadratic function, the values of x that give y=0.