Heres what I did so far:
Plug 3c in for k:
I differentiate, set equal to 0, and solve for x to get:
However this is not the correct answer.... Any help would be greatly appreciated. Thanks!
I tried playing around with it and factoring out the (d-x)^3
And from here I'm lost once again... Do i set a lowest common denom now or am I completely off track...