Prime factor is a factor that is a prime number.
Prime factors of :
8 : 2 × 2 × 2. Also can be written as

12 : 2 × 2 × 3. Also can be written as

20 : 2 × 2 × 5. Also can be written as

30 : 2 × 3 × 5
56 : 2 × 2 × 2 ×7. Also can be written as

70 : 2 × 5 × 7
You can find all of them by using Factor Tree [ as shown in the picture] or Dividing by prime numbers.
A minute less than 1 hour, to be exact.
The answer is 42 but the greatest common factor is 3
The answer should be -2y-6