Logistic Model with Harvesting

If the growth of a population follows the logistic model but is subject to "harvesting" (such as hunting or fishing), the model becomes

where is the rate of harvesting.

(a) Suppose that the harvest rate is constant. Determine the maximum sustainable harvest.

Hint: Set and use the quadratic formula to find

I've tried using the hint and setting the equation to 0 then using quad. form., but haven't had any luck on this. If any one could point me in the right direction on what to do, I'd really appreciate it.

What I have is:

But then it says to use for quadratic formula, so I'm not sure if I should be solving for h or what..