y = a ( x - h )^2 + k The k represents the Y coordinate of the Vertex. Also known as the Optimal value. The optimal value is the lowest or highest value in the parabola. The axis of symmetry is always written like y= optimal value. This what a optimal value looks like:
This is an example of a question asking for the optimal value: Find the optimal value of this vertex form equation y=2(x+3)+9. The optimal value of this equation is y=9 because that is the k value in the equation.