You need to remember that torque, angular acceleration and moment of inertia are related by `tau = alpha*I` .
You should notice that the initial angular velocity is `omega_1 ` = 50 rev/s and the final angular velocity is `omega_2` = 0 (rest).
You need to convert 50 rev/s in rad/s such that:
`omega_1 = 50*2pi = 314 rad/s`
You need to calculate the angular acceleration such that:
`alpha = (omega_2 - omega_1)/t = (0-314)/60`
Notice that the time is expressed in seconds and the given period of slowing down is of 1 minute = 60 seconds.
`alpha = -5.23`
Considering the shape of turbine as the shape of a solid wheel yields:
`I = MR^2/2 = 1200*0.8^2/2 = 600*0.8*0.8 = 384 Kgm^2`
Hence, `tau` = -5.23*384 = -2008.32Nm
Hence, evaluating the braking torque needed for slowing down the turbine from 50 rev/s to rest yields `tau` = -2008.32Nm.