We have that the unit cost is given by a parabola, and the leading coefficient is given by 0.6 (it is positive), then, we have a minimum in the shape of the parabola.
A way to find the minimum is to find the vertex of the parabola (in this given case, the vertex is the minimum point) which coordinates are as follows:
[tex]x_v=-\frac{b}{2a},y_v=c-\frac{b^2}{4a}[/tex]
Since we need to find the minimum unit cost, we need to find the value for y. Then, we have:
[tex]0.6x^2-324x+56258[/tex]
a = 0.6
b = -324
c = 56258
Then
[tex]y_v=56258-\frac{(-324)^2}{4\cdot(0.6)}\Rightarrow y_v=12518[/tex]
Therefore, the minimum unit cost is equal to $12,518.