The next problem involves slightly more calculation than the discussion above.
1) calculate total moles, 2) calculate total volume, 3) divide moles by volume to get molarity
You can also think of it this way: M1V1 + M2V2 = M3V3
Where the '1' refers to one starting solution, '2' to the other starting solution and '3' refers to the mixed solution (hints: V3 is the total volume after mixing and M3 is almost always the unknown)
2. Calculate the final concentration if 2.00 L of 3.00 M NaCl and 4.00 L of 1.50 M NaCl and 4.00 L of water are mixed. Assume there is no volume contraction upon mixing.