#### Answer

$32y^{14}$

#### Work Step by Step

Use the product property for exponents $(a^{m} \times a^{n}=a^{m+n})$, as well as the associative and commutative property, to simplify $8y^{5}\times 4y^{9}$:
$8y^{5}\times 4y^{9}$
=$(8\times 4)(y^{5}\times y^{9})$
=$(32)(y^{5+9})$
=$32y^{14}$