Answer: 12 m/s

Given $x = t^3 - 3t^2 + 2t$.

Average Velocity $v_{\text{avg}} = \large\frac{\text{displacement}}{\Delta \text{time}}$

$\Delta$ time $= 4s - 2s = 2s$

Position at $t = 2s \rightarrow x(2) = 2^3 - 3\times 2^2 + 2\times2 = 0$

Position at $t = 4s \rightarrow x(4) = 4^3 - 3\times 4^2 + 2\times4 = 24\;m$

$\Rightarrow$ Displacement $ = 24 - 0 = 24\;m$

$\Rightarrow$ Average velocity $ = \large\frac{24}{2} $$ = 12\;m/s$