Answer
See explanation.
Work Step by Step
a. $m=\frac{E}{c^2}=\frac{1.0\times10^{20}J}{(3.00\times10^8m/s)^2}=1.11\times10^3 kg$
b. Find the volume V.
$$\rho=\frac{m}{V}$$
$$V=\frac{m}{\rho}=\frac{1.11\times10^3 kg }{7860kg/m^3}=0.141m^3$$
Find the side length of a cube with this much volume.
$$V=L^3$$
$$L=V^{1/3}=(0.141m^3)^{1/3}=0.521m$$
