https://www.youtube.com/watch?v=TCcSZEL_3CQ
Lagrange's theorem states that order of every subgroup of G, it must be the divisor of G.
So the largest subgroup will be 84 which is trivial, but in the question it is asking for the proper subgroup hence it will be 42.
Reference: https://en.wikipedia.org/wiki/Subgroup
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