A smokestack deposits soot on the ground with a concentration inversely proportional to the square of the distance from the stack. With two smokestacks $d$ miles apart, the concentration of the combined deposits on the line joining them, at a distance $x$ from one stack, is given by $$S = \frac{c}{x^2} + \frac{k}{(d-x)^2}$$ where $c$ and $k$ are positive constants which depend on the quantity of smoke each stack is emitting. If $k = 3 c$, find the point on the line joining the stacks where the concentration of the deposit is a minimum.

$x_{\hbox{min}}$ = mi