Answer
$t=5.0s$
Work Step by Step
We know that
$\alpha=\frac{\omega-\omega_{\circ}}{t}$
This can be rearranged as:
$t=\frac{\omega-\omega_{\circ}}{\alpha}$
We plug in the known values to obtain:
$t=\frac{-5.0-5.40}{-2.10}$
$t=5.0s$
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