The molar concentration of a solution of a compound is the number of moles of the compound present in one liter of the solution.
It is required to make a 0.045 M solution of magnesium chloride. This implies that one liter of the final solution contains 0.045 moles of magnesium chloride. This solution has to be prepared by adding a volume V of a solution of magnesium chloride with a molarity M to 4.2 moles of magnesium chloride. There are unlimited combinations of the molar concentration of the solution that is used and its volume.
One option is to use a volume of pure water. To determine the required volume V of water required, solve 4.2/V = 0.045
=> V = 4.2/0.045
=> V = 93.33 liters
If 93.33 liters of pure water is added to 4.2 mole of magnesium chloride the resulting solution has a molarity of 0.045.