CPM Homework Help : CCA2 Problem 10-10

If the third term of an arithmetic sequence is $13$ and the seventeenth term is $-29$ , what is the eighth term?

Hint 1:

Create and solve a system of equations based on the sequence $t(n)=an+b$ :
$13=a(3)+b$
$-29=a(17)+b$

Hint 2:

Use the Elimination Method to solve for $a$ and $b$ .

Partial Answer:

$t(8)=(8)(a)+b=?$