If I understand your question correctly, you want to take these roots and find the quadratic associated with them. You can set these roots equal to 0 as follows
x-8=0
x+8=0
Then multiply the two factors using the distributive property or foil
(x+8)(x-8)=
x^2-8x+8x-64
simplify to x^2-64
In the graph you will see that the parabola crosses the x axis at 8 and -8 which is why x=8 and x=-8