This is a simple dilution problem. The way you wold go about solving this would be:
M of solute before dilution X L of solution before dilution = M of solute after dilution X L of solution after dilution.
For this problem the numbers would be:
6M X .003 Liters = n X .006 liters, solve for n.
This works because Molarity = Moles of solute/ Liters of Soln, and you can rewrite this equation as Liters as solution X Molarity= Moles of solute. And since the number of moles stays the same before and after the dilution, you can use this equation.
M of solute before dilution X L of solution before dilution = M of solute after dilution X L of solution after dilution.
For this problem the numbers would be:
6M X .003 Liters = n X .006 liters, solve for n.
This works because Molarity = Moles of solute/ Liters of Soln, and you can rewrite this equation as Liters as solution X Molarity= Moles of solute. And since the number of moles stays the same before and after the dilution, you can use this equation.