Answer:
x+3
Step-by-step explanation:
21=7x3. Hence answer is 3
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.
The first one is B and the second one is C
The constant of proportionality would be "4" ... proportional relationships are in the form of y = kx.. so in your ordered pairs, you can see that to get the 'y' coordinates you would have to multiply the 'x' coordinates by 4.