We need to minimize the distance between the points and (c, 0).

The distance between two the two points is:

Note that if we square the distance:

we still find the minimum x in the same place with a much easier computation.

So now take your x derivative with respect to the equation, set it equal to 0, etc.

Not sure why you've got the part, though.

Edit: Ahhhh, because the domain of the function is only positive x.

-Dan