Answer:
[tex]x= -\dfrac {9}2 +\dfrac{\sqrt{57}}{2},~~x = -\dfrac {9}2 - \dfrac{\sqrt{57}}{2}[/tex]
Step-by-step explanation:
[tex]~~~~~~~x^2 +9x +12 = 6\\\\\implies x^2 +9x = 6-12\\\\\implies x^2 +9x = -6\\\\\implies x^2 +2 \cdot \dfrac 92 \cdot x +\left(\dfrac 92\right)^2 -\left( \dfrac 92 \right)^2 = -6\\\\\implies \left(x+ \dfrac 92 \right)^2 -\dfrac{81}{4}= -6\\\\\implies \left(x +\dfrac 92 \right)^2 = -6 + \dfrac{81}4\\\\\implies \left(x + \dfrac92 \right)^2 = \dfrac{57}{4}\\\\\implies x+ \dfrac 92 = \pm\sqrt{\dfrac{57}4}\\\\\implies x = -\dfrac {9}2 \pm\dfrac{\sqrt{57}}{2}[/tex]
