\[\left( - b^{3} \right)^{5} \cdot \left( - b^{5} \right)^{7}\ :b^{25} =\]
\[= - b^{15} \cdot ( - b)^{35}\ :b^{25} =\]
\[= b^{15 + 35 - 25} = b^{25}\]