Find the radius of an aluminum cylinder that is 2.00 cm long and has a mass of 12.4 g.

find the volume of the cylinder using the mass and its density V = M / D
so Volume = 12.4 / 2.7 = 4.59259259259cm3 

Divide by length  to get the volume of a 1cm slice: 4.59259259259 / 2 = 2.2962962963

The cross section area in cm2 = the volume in cm3.  

Reverse pi r^2 to get the radius, so divide by pi then take the square root. 

R = √V / R = 0.85494667244cm 

hope that helps