Answer
$t=3.00 sec$
Work Step by Step
The formula to be used here is:
$\omega_f=\omega_{\circ}+\alpha t$
We then rearrange this formula to make $t$ the subject of the formula: $t=\frac{\omega_f-\omega_{\circ}}{\alpha}$
Putting the values in the formula and solving:
$t=\frac{0-12.60}{-4.20}$
$t=3.00 sec$
