Answer:
[tex]3(1+7a+5b)[/tex]
Step-by-step explanation:
The distributive law states that
[tex]a\cdot (b+c)=a\cdot b+a\cdot c.[/tex]
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
[tex]3+21a+15b=3(1+7a+5b).[/tex]
