The formula to be used here is that of density which is shown below
Density = Mass ÷ volume
The <u>density</u> of the rubbing alcohol (also known as isopropyl alcohol) is given in the question as 0.79 g/ml while the <u>volume</u> of same is given as 18.7ml. The mass of this rubbing alcohol is the unknown (which we can derive from the formula given earlier).