Answer
$-\frac{1}{2}y^3$
Work Step by Step
Divide the monomials by cancelling out common factors of the numbers and subtracting the exponents of variables.
$\frac{-9y^8}{18y^5}=-\frac{9}{18}y^{(8-5)}=-\frac{1}{2}y^3$
Check the answer.
$18y^5(-\frac{1}{2}y^3)=-\frac{18}{2}y^{5+3}=-9y^8$
