Chapter 9 - Sequences and Series - 9-2 Arithmetic Sequences - Practice and Problem-Solving Exercises - Page 577: 68

$a_1=-4$ $d=4$

With $a_4=8$, the value of $a_7$ can be found by adding the common difference $d$ three times. Since $a_4=8$, then \begin{align*} a_7&=a_4+3d\\\\ 20&=8+3d\\\\ 20-8&=3d\\\\ 12&=3d\\\\ \frac{12}{3}&=\frac{3d}{3}\\\\ 4&=d \end{align*} The value of $a_1$ can be found by subtracting $d$ from $a_4$ three times. Thus, \begin{align*} a_1&=a_4-3d\\ a_1&=8-3(4)\\ a_1&=8-12\\ a_1&=-4 \end{align*}