Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 32

$56x^{2}y+21xy^{2}$

Using the Distributive Property, the product of the given expression, $ 7xy(8x+3y) ,$ is \begin{array}{l}\require{cancel} 7xy(8x)+7xy(3y) \\\\= 56x^{1+1}y+21xy^{1+1} \\\\= 56x^{2}y+21xy^{2} .\end{array}