Simplify each expression.
$(-5a^2)\left(3a^8\right)$

Simplify each expression using the product rule if possible.

${12}^3\cdot {12}^9$

Simplify each expression using the product rule if possible.

$(-4{)}^6(-4)$

Simplify each expression.

$xy\cdot 3x^2$

Simplify each expression.

$\left(2s^{3}t\right)\left(3s^4{t}^2\right)$