= Solution
A straight line in $C/T$ against $T^2$ means $C/T=\gamma+\beta T^2$. Reading its intercept and slope gives approximately $\gamma\simeq2.1\,\mathrm{mJ\,mol^{-1}K^{-2}}$ and $\beta\simeq2.7\,\mathrm{mJ\,mol^{-1}K^{-4}}$; the graphical precision is only a few percent. Hence the molar <heat capacity> is approximately
$$
\boxed{C(T)\simeq(2.1\times10^{-3}\,\mathrm{J\,mol^{-1}K^{-2}})T+(2.7\times10^{-3}\,\mathrm{J\,mol^{-1}K^{-4}})T^3.}
$$
The linear <electronic heat capacity> comes from conduction <electrons> within <energy> of order $k_BT$ of the <Fermi energy>; the <Pauli exclusion principle> prevents deeply occupied states from contributing freely. The cubic lattice term is the low-temperature <Debye heat capacity> of three-dimensional acoustic <phonons>, whose low-frequency <density of states> is proportional to <frequency> squared. These are molar coefficients, so the <temperature> powers carry the extra kelvin factors indicated.
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