The formula for the volume of a cone is (1/3)*area of the base * height. The base is a circle, whose area is pi times radius^2. Here this is pi * (r^2) = 3.14*(3mm)^2 = 28.26 mm^2. And so the volume is (1/3) * 28.26 mm^2 * 7mm = 65.94 mm^3. I recommend that you find the most common formulas of the geometrical figures and make a flashcard that you can use at any moment.