Final answer:
To find the vertex, you need to plug the x-coordinate obtained from the formula -b/2a into the original equation.
Step-by-step explanation:
The value obtained from the formula -b/2a is the x-coordinate of the vertex of a quadratic equation. This value represents the axis of symmetry of the parabola. To find the vertex, you need to substitute this x-coordinate back into the original equation to find the corresponding y-coordinate. Therefore, the correct option is C: Plug it into the original equation.