Answer :

[tex]\frac{3b}{b+2}+\frac{12}{b+2}=\frac{3b+12}{b+2}\\\\\\\frac{3b+12}{b+2}=\frac{3b+6+6}{b+2}=\frac{3b+6}{b+2}+\frac{6}{b+2}=\frac{3(b+2)}{b+2}+\frac{6}{b+2}=3+\frac{6}{b+2}[/tex]