according to the solutions from the textbook, the correct answer is:

TNTeq = 0.535 kg.

How? please thoroughly workout and explain this problem step by step, because i do not understand the textbook's explanation.

include any energy and/or entropy balance (if needed) and any other details

High-pressure helium is available from gas producers in 0.045m3 cylinders at 400 bar and 298K. Calculate the explosion equivalent of a tank of compressed helium in terms of kilograms of TNT. Assume helium is an ideal gas