One cubic meter (1.0 m3
) of aluminum has a
mass of 2700 kg, and a cubic meter of iron has
a mass of 7860 kg.
find the radius of a solid aluminum sphere
that has the same mass as a solid iron sphere
of radius 4.91 cm

Because the masses that you give are for blocks that are 1 cubic meter in volume, they also serve as the densities for the two metals that you are comparing. 

mass = density*volume 

volume = (4/3)*pi*r^3 

volume of iron sphere = (4/3)*3.14*0.0201^3 = 3.40*10^-5 m^3 

mass of iron sphere = 7860* 3.40*10^-5 m^3 = 0.27 kg = mass of Aluminum Sphere 

Volume of Al Sphere = 0.27/2700 = 9.90*10^-5 m^3 

Radius = cube root (volume / (4/3) / pi) = 2.87 cm. 

I did this using the MS calculator, and I'm not 100% sure on the numerical answer, but the process is what you need to do to solve the problem. You should double check my answer.

hope this helped :)