What is the optimal value of a parabola?
By optimal, I assume you are looking for the maximum or minimum.
(The optimal shape for a parabolic reflector, used for telescopes, satellite receivers, etc... is a different subject)
The maximum or minimum occurs at the vertex of the parabola.
If the graph opens down (leading coefficient is negative), then the function has a maximum. If the graph opens up (the leading coefficient is positive) there is a minimum.