Answer
$\frac{-7x^2}{y^3}$; Bob treated $-7$ as a negative exponent and incorrectly moved it to the denominator.
Work Step by Step
Bob treated $-7$ as a negative exponent and incorrectly moved it to the denominator. Negative numbers that lack exponents should remain where they are.
$-7x^2y^{-3}$
$\frac{-7x^2}{y^3}$
