Chapter 1 - Foundations for Algebra - 1-1 Variables and Expressions - Practice and Problem-Solving Exercises - Page 7: 26

$2.1(5+k)$

"The product of 2.1 and the sum of $5$ and $k$." We must first find the sum of $5$ and $k$. Since sum of means to add, we add the terms, giving us $5+k$ We now must multiply this to 2.1, as "the product of" means the same thing as multiplication. We are left with the answer, $2.1(5+k)$