Planck's theory of quantized oscillators leads to an average energy $$E = \frac{W}{e^{W/kT}-1}$$ where $W$ and $k$ are constants.

Approximate $E$ with the first two nonzero terms of the Taylor polynomial when $kT >> W$ (i.e. in the nonrelativistic scenario):
$E\approx$ .

(Hint: If $x >> y$, then $\frac{y}{x} \approx 0$.)