Chapter 1 - Introduction to Algebraic Expressions - 1.6 Subtraction of Real Numbers - 1.6 Exercise Set - Page 49: 23

$\frac{11}{3}$

The opposite of a real number $a$ is $-a$. For any two numbers $a$ and $-a$, $a+(-a)=0$. That is, the sum of a number and its opposite is equal to 0. Therefore, the opposite of $-\frac{11}{3}$ is $\frac{11}{3}$, because $-\frac{11}{3}+(\frac{11}{3})=\frac{11}{3}-\frac{11}{3}=0$.