Answer
$=18x^{9}y^{5}$
Work Step by Step
When multiplying two exponential expressions, group factors with the same base: $$(-9x^{3}y)(-2x^{6}y^{4})$$
$$=(-9\times(-2))(x^{3}\times x^{6})(y\times y^{4})$$
$$=18x^{(3+6)}y^{(1+4)}$$
$$=18x^{9}y^{5}$$
