[tex]x^2+3x-18=0\\\\a=1;\ b=3;\ c=-18\\\\\Delta=b^2-4ac\to\Delta=3^2-4\cdot1\cdot(-18)=9+72=81\\\\x_1=\frac{-b-\sqrt\Delta}{2a};\ x_2=\frac{-b+\sqrt\Delta}{2a}\\\\\sqrt\Delta=\sqrt{81}=9\\\\x_1=\frac{-3-9}{2\cdot1}=\frac{-12}{2}=-6;\ x_2=\frac{-3+9}{2\cdot1}=\frac{6}{2}=3[/tex]
