This is really difficult for me. if someone could me i would really appreciate it.

Since we can add up the factors of 28 (excluding 28 itself) to get
1+2+4+7+14 = 28
this means that 28 is a perfect number. In the interval from 20 to 30, it is the only perfect number.

28

Note: Something like 22 is not a perfect number since
1+2+11 = 14
which isn't 22 like we want. Every number, in the interval from 20 to 30, but 28 has this problem. 

It’s going to be d. 4x^2-4x-19’