Multiply & divide rational expressions (advanced)

The following equation is true for all real values of $z$ for which the expression on the left is defined, and $B$ is a polynomial expression. >$\dfrac{5z^2}{z^2+5z} \div \dfrac{B}{z^2-2z-35}=1$ **What is $B$?** $B=$ [[☃ expression 1]]