Hi Sukham,
Thanks for your email. I have attached a solution here, please check its veracity (I don't have the solutions either so I hope it's correct).
The reason that I would use HEW rather than the volume of NAG3 is because the passage only gives you the volume and number of moles for a single injection, and multiple injections are needed for this titration.
You could figure out how many injections it takes to get to the equivalence point, but it's much easier to just look at the graph and realize HEW and NAG3 are in a 1:1 molar ratio at the equivalence point, so you can use the moles HEW = moles NAG3, which is easiest to get through this moles HEW given in the passage.
Please let me know if this is unclear,
Thanks,
Katt