The square root is between 8 and 9 so some of the prime factors must be less than 9.
75 is an odd number so 2 can't be a factor, 3? yes, 3 x 25. Aha, 25 = 5 x 5 so prime factorisation is 3 x 5 x 5. Composite factors can easily be worked out as there are only two different prime factors...
The composite factors of 75 are 15,25 and 75.
The factors of 75 are: 1 3 5 15 25 75 The prime factors are: 3, 5
75's factors are: 1,3,5,15,25,75 so it has 6 factors
The common factors of 75 and 375 are 1, 3, 5, 15, 25 and 75.
The composite factors of 75 are 15, 25, 75