Answer:

Step-by-step explanation:
The distributive law states that

The expression 3+21a+15b consists of three terms:
- 3=3·1;
- 21a=3·7a;
- 15b=3·5b.
You can see that 3 is the common factor of these three terms, thus

For number 3, it is reciprocal
and for number 4, it is whole number