How do I calculate quantity to minimize long-run average total cost? I have a formula for the long-run total cost curve,

$$TC(Q) = 6000Q + 40Q^2 + Q^3$$

and I'm trying to find the quantity that minimizes the long-run average total cost. 

I assume I'm trying to find the value $Q$ (quantity) that produces the total cost ($TC(Q)$) where $\frac{TC(Q)}{Q}$ is the lowest. But how do I do this short of trial and error?
Your are right. You have to minimize the average cost.

$$c(Q)=\frac{C(Q)}{Q}=6000 +40Q+Q^2$$

Calculate the first derivative and set it equal to zero:

$ c'(Q)=40+2Q=0 $

Solve this equation for $Q$. Denote the optimal value as $Q^*$. $Q^*$ can be a local maximum or a local minimum

If $c''(Q^*)&gt;0$, then you have found the local minimum.

The local minimum is also the absolute minimum, because

$$\lim_{Q \to \infty } 6000 +40Q+Q^2=\infty$$

$$\lim_{Q \to -\infty } 6000 +40Q+Q^2=\infty$$