Chapter 5 - Section 5.4 - Polynomials in Several Variables - Exercise Set - Page 375: 39

$7x^2+38xy+15y^2$

Using $(a+b)(c+d)=ab+ad+bc+bd$ or the FOIL Method, the product of the given expression, $ (x+5y)(7x+3y) ,$ is \begin{array}{l}\require{cancel} x(7x)+x(3y)+5y(7x)+5y(3y) \\\\= 7x^2+3xy+35xy+15y^2 \\\\= 7x^2+38xy+15y^2 .\end{array}