it finds the interception with the axis ' ' x' ' co-ordinated, and with the axis ' ' y' ' , that is what you look for
y=x[sup:1oultpnb]2[/sup:1oultpnb]-7
y=0
x[sup:1oultpnb]2[/sup:1oultpnb]-7=0
x[sup:1oultpnb]2[/sup:1oultpnb]=7
x=+-root(7)
if x=0
y=-7 minimum
you most see your book
