Chapter 4 - Polynomials - 4.7 Polynomials in Several Variables - 4.7 Exercise Set - Page 284: 46

$10x^2-13xy-3y^2$

Using $(a+b)(c+d)=ac+ad+bc+bd$ or the product of $2$ binomials, the given expression, $ (5x+y)(2x-3y) ,$ simplifies to \begin{array}{l}\require{cancel} 5x(2x)+5x(-3y)+y(2x)+y(-3y) \\\\= 10x^2-15xy+2xy-3y^2 \\\\= 10x^2-13xy-3y^2 .\end{array}