$\left( - 9x^{4}\right)\left( - 3x\right)$
Rewrite using the commutative property of multiplication$ - 9 \cdot - 3x^{4}x$
Move $x$$ - 9 \cdot - 3\left(x \cdot x^{4}\right)$
Raise $x$$ - 9 \cdot - 3\left(x^{1}x^{4}\right)$
Use the power rule $a^{m}a^{n} = a^{m + n}$$ - 9 \cdot - 3x^{1 + 4}$
Add $1$$ - 9 \cdot - 3x^{5}$
Multiply $ - 9$$27x^{5}$