$$ x^2 -6x = 9 \quad | -9 $$
$$ x^2-6x-9=0 $$
Anwenden der PQ Formel
$$ p=-6 \quad q=-9 $$
$$ \frac{-p}{2}\pm \sqrt{\left(\frac{p}{2}\right)^2-q} $$
$$ \frac{--6}{2}\pm \sqrt{\left(\frac{-6}{2}\right)^2-(-9)} $$
$$ x_1 = -1.2426... \quad x_2 =7.2426 $$
~plot~ x^2-6x-9 ~plot~
Hope this helps!