#### Answer

$-35x^{5}y^{5}$

#### Work Step by Step

We use the rule $a^{m}\times a^{n}=a^{m+n}$ in the simplification of the expression:
$(-5x^{2}y)(7x^{3}y^{4})$
=$(-5\times7)\times(x^{2}\times x^{3})\times(y\times y^{4})$
=$(-35)\times(x^{2+3})\times(y^{4+1})$
=$(-35)\times(x^{5})\times(y^{5})$
=$-35x^{5}y^{5}$