Question

Can someone explain why (a+1)b-3(a+1)=(a+1)(b-3)

Answer 1

it is the distributive law

Answer 2

\[\large \color{red}{A}(\color{blue}B+\color{green}C)=\color{red}A\color{blue}B+\color{red}A\color{green}C\]
where
\[\color{red}A=\color{red}{a+1}\]\[\color{blue}B=\color{blue}b\]\[\color{green}C=\color{green}{-3}\]