The net torque, #tau#, can be expressed in terms of the moment of inertia and angular acceleration, where #tau=Ialpha#.

Assuming that this is a thin, rigid rod, with the axis of rotation through its center, the moment of inertia, #I#, of the rod is given by #1/12ML^2#, where #M# is the mass of the rod and #L# is its length. If the rod is rotated about its end, #I=1/3ML^2#. I will show the calculation for the axis of rotation through the center. We are given both #L# and #M#. Thus,

#I=1/12*3kg*(4m)^2#

#I=4kgm^2#

The angular acceleration of the rod as it rotates, #alpha#, can be calculated from the given values of time and frequency, where #alpha=(Δomega)/( Δt)#. We can find #omega# from the given frequency of #5Hz#, as #omega=2pif#.

#omega=2pi(5s^-1)#

#omega=10pi(rad)/s#

Because the problem states that the frequency *changed* by this amount, this is # Δomega#. We are given that this took place over #1s#, which is our # Δt# value. Thus,

#alpha=(10pi(rad/s))/(1s)#

#alpha=10pi(rad)/s^2#

Now that we have all of the values we need, we can calculate the torque.

#tau=Ialpha#

#tau=4kgm^2*10pi(rad)/s^2#

#tau=40piNm#

#tau≈126Nm#