Your code has a huge flaw in that you aren't outputting what is being calculated. You are outputting the index of the item being multiplied, not the actual value being multiplied.
publicstaticList<string>Format(List<int>list)=>list.AsParallel().Select(Extensions.MultiplyBy2).Zip(list.AsParallel(),(result,item)=>$"{item} x 2 = {result}").ToList();
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Your code has a huge flaw in that you aren't outputting what is being calculated. You are outputting the index of the item being multiplied, not the actual value being multiplied.