M1 = initial molarity;

V1 = initial volume;

M2 = final molarity;

V2 = final volume;

M1V1=M2V2 is normally to work out the concentration or volume of the concentrated or dilute solution. Hence, you can utilize the formula M1V1=M2V2 when trying to calculate from the initial molarity, initial volume, final molarity, or final volume (if you are provided with at least three of the following, it is a clear indicator that you are required to use this formula if you need to get the missing value).