You should probably find the roots by using this formula. For a quadratic equation $ax^{2}+bx+c$ we have the root's as: $x = \frac{-b \pm \sqrt{D}}{2a}$
where $D$ is the discriminant given by $D=b^{2}-4ac$. So in your case note that $a=1$, $b=6$ and $c=7$. Hope you can find your way from here. So $D=6^{2}-4\times 7 \times 1=8$. So you have the value of the roots as: $ x= \frac{-6 \pm \sqrt{8}}{2 \times 1} = \frac{-6 \pm 2\sqrt{2}}{2} = -3 \pm \sqrt{2}$
For more information regarding Quadratic Equations Please see:
Here is the Image of how your parabola will look like:
