Answer
The magnitude of the average torque is $~~1.47~N\cdot m$
Work Step by Step
We can find the torque on the flywheel:
$\tau~t = \Delta L$
$\tau = \frac{\Delta L}{t}$
$\tau = \frac{L_f-L_i}{t}$
$\tau = \frac{(0.800~kg~m^2/s)-(3.00~kg~m^2/s)}{1.50~s}$
$\tau = -1.47~N\cdot m$
The magnitude of the average torque is $~~1.47~N\cdot m$
