Confused In Modular Exponentiation

#1

I get that what I need to do in question but I am unable to figure out what am I doing wrong in my solution can someone please give some hint.

#2

@Prakhar2810
You are doing a mathematical mistake:

(a1 + a2 + a3) % m = (((a1 % m) + (a2 % m)) % m + (a3 % m)) % m;

#3

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