Algebra 1: Common Core (15th Edition)

$-3b-21$
The distributive property allows us to distribute anything multiplied by something inside of the parenthesis to whatever is inside of the parenthesis. Thus, we distribute the negative 3 to everything inside the parenthesis, giving us an answer of $-3b-21$. (A key rule to remember is that subtracting a negative is the same as addition, so b-(-7) is the same as b+7).