A man launches his boat from point A on a bank of a straight river, 3 km wide, and wants to reach point B, 3 km downstream on the opposite bank, as quickly as possible. He could row his boat directly across the river to point C and then run to B, or he could row directly to B, or he could row to some point D between C and B and then run to B. If he can row 6 km/h and run 8 km/h, how many kilometers downstream from C should he aim for with his boat? (Assume that the speed of current is negligible).