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, from one stack, is given by

S=(c/x^2)+(k/((d-x)^2))

where andare positive constants which depend on the quantity of smoke each stack is emitting. If=6, find the point on the line joining the stacks where the concentration of the deposit is a minimum.