Power is torque times the angular frequency, so I'll find the angular frequency first:
Next, I get the stress-concentration factor from Figure 3.32 on page 167:
So K = 1.55. Since the allowable shearing stress is 45 MPa, in the filleted region as well as in the body of the shaft, I divide this by 1.55 to get an average allowable stress of 29.03 MPa. Next, I find the torque:
The power is this torque times the angular frequency, or 269 kW.