\(x^2-9x+15\)
\(=x^2-2.x.\dfrac{9}{2}+\dfrac{81}{4}-\dfrac{21}{4}\)
\(=\left(x-\dfrac{9}{2}\right)^2-\left(\dfrac{\sqrt{21}}{2}\right)^2\)
\(=\left(x-\dfrac{9}{2}+\dfrac{\sqrt{21}}{2}\right)\left(x-\dfrac{9}{2}-\dfrac{\sqrt{21}}{2}\right)\)
\(=\left(x+\dfrac{\sqrt{21}-9}{2}\right)\left(x-\dfrac{9+\sqrt{21}}{2}\right)\)