\[b(3b + 1)(2b - 5) =\]
\[= b\left( 3b \cdot 2b + 3b \cdot ( - 5) + 2b - 5 \right) =\]
\[= b\left( 6b^{2} - 15b + 2b - 5 \right) =\]
\[= b\left( 6b^{2} - 13b - 5 \right) =\]
\[= 6b^{2 + 1} - 13b^{1 + 1} - 5b =\]
\[= 6b^{3} - 13b^{2} - 5b.\]