Hello, I'm having some brain freeze on finding the derivative for this problem.

First, I changed the denominator to , and then used the Product Rule.

That left me with:

This is where I'm getting brain frozen. I checked the back off the book it says the answer is: .

I'm not sure where the value of came from. I see parts of the Chain Rule, but the 128.772 has me dumbfounded. If there's a coefficient in front of what happens when taking the derivative of ?