(a) Energy / volume = $\frac{1}{2}\ \times stress\ \times strain\ $
= $\frac{1}{2}Y \times strain \times strain = \frac{1}{2}Y\ \times strain^{2}$
= $\frac{1}{2} \times 2 \times 10^{10} \times 0.06\ \times 10^{- 2} \times 0.06\ \times 10^{- 2}$
= $3600\ J\ m^{- 3}$