Answer
The rate at which the angular momentum is changing is $~~(56~kg~m^2/s^2)~\hat{k}$
Work Step by Step
In part (b), we found that the torque about the origin is $~~(56~N\cdot m)~\hat{k}$
We can find the rate at which the angular momentum is changing:
$\frac{dl}{dt} = \tau = (56~N\cdot m)~\hat{k}$
The rate at which the angular momentum is changing is $~~(56~kg~m^2/s^2)~\hat{k}$
