The illumination at a distance d from a light source with intensity I is given by kI/d^2 where k is some positive constant. Two light sources, one with an intensity of I1=3 candelas (a candela is a unit of light intensity) and the other with an intensity of I2=9 candelas are 5 feet apart. At what point between these two light sources will the sum of their illumination be minimized? give answer as a distance from the first light source.