Chapter 8 - Polynomials and Factoring - 8-1 Adding and Subtracting Polynomials - Practice and Problem-Solving Exercises - Page 478: 22

$5x^{2}$

In order to add monomials, we combine like terms. Like terms are terms that have the same variable raised to the same power. Note, by this definition, numbers like 10 and 15 are also like terms as they both have a degree of 0. Thus, we combine like terms to obtain the answer: $5x^{2}$