Heres what I did so far:

Plug 3c in for k:

$\displaystyle S = c/x^2 + 3c/(d-x)^2$

I differentiate, set equal to 0, and solve for x to get:

$\displaystyle 0=-c(2x)+-3c[2(d-x)(-1)]$

$\displaystyle 0=-2cx+6cd-3cx$

$\displaystyle 0=-5cx+6cd$

$\displaystyle x=6d/5$

$\displaystyle x min = 6d/5 miles$

However this is not the correct answer.... Any help would be greatly appreciated. Thanks!