Answer
$\tau = 1.5\times 10^4~m\cdot N$
Work Step by Step
We can find the angular acceleration:
$\alpha = \frac{\Delta \omega}{t} = \frac{0.68~rad/s}{34~s} = 0.020~rad/s^2$
We then find the moment of inertia:
$I = \frac{1}{2}MR^2 = \frac{1}{2}(31,000~kg)(7.0~m)^2$
$I = 759,500 ~kg\cdot m^2$
We can then use this angular acceleration and the moment of inertia to find the torque:
$\tau = I \alpha$
$\tau = (759,500 ~kg\cdot m^2)(0.020~rad/s^2)$
$\tau = 1.5\times 10^4~m\cdot N$