2. Thus, $\sigma = (1,2,3,4)$ and $\tau = (2,4)$. A simple calculation will show $\tau \sigma = \sigma^3 \tau$. But $\sigma^3 = \sigma^{-1}$. Thus, $\tau \sigma = \sigma^{-1} \tau$. This means, $\tau \sigma^{k} = \tau \sigma \sigma^{k-1} = \sigma^{-1} \tau \sigma^{k-1} = \sigma^{-1} \tau \sigma \sigma^{k-2} = \sigma^{-2} \tau \sigma^{k-2} = ... = \sigma^{-k} \tau$.