Actually, Newton-meter or N-m is equivalent to Joules. On the otherhand, watts in simplified units, is Joules/second. So, first, convert J/min to J/s.
24,000 J/min (1 min/60 s) = 400 J/s = 400 W
Since the efficiency is 70%, then the output is:
Output = (400)(0.7) = 280 Watts