2 of the 8 factors are 8 and 28.
So the Unknown factors are
__,___,____,___,___,____,8,28.

From part (a)
We can see that the factors of 126 are
1,2,3,6,7,18,21,42,63,126

We can infer from part (a) that 1 and its number are the factors of the number. So we can safely say that 1 is among the 8 factors.
1,___,___,___,___,___,8,28

We can also see that factors of 63 includes 3 and 21 as well as factors of 42 are 6 and 7 and factors of 21 are 3 and 7.

What we can say about the factors is that. Factors of the particular factor are also the factors of the number.

So factors of 8 are 1,2,4,8
28 are 1,2,4,7,14,28
So we can say that the factors are
1,2,4,7,8,14,28,___

However now we only have 7 factors.
Factors are arranged in the way such that 1 factor multiply by last factor would give us the main number. And the second factor multiply by the second last factor would give us the same number.
Hence, 28 x 2 = 56.
And therefore your 8 factors are
1,2,4,7,8,14,28,56