Chapter 9 - Sequences and Series - 9-2 Arithmetic Sequences - Practice and Problem-Solving Exercises - Page 577: 66

The other term is $8$.

Recall: Arithmetic mean (or average) of two numbers $a$ and $b$ is $\dfrac{a+b}{2}$. Let $a$ = -20 $b$ = the other term Using the formula above gives: \begin{align*} \text{arithmetic mean} &= \dfrac{a+b}{2}\\\\ -6&=\dfrac{-20+b}{2}\\\\ 2(-6)&=2\left(\dfrac{-20+b}{2}\right)\\\\ -12&=-20+b\\\\ -12+20&=b\\\\ 8&=b \end{align*}