Find the sequence that corresponds to the terms of the series.

$t\left(5\right) = 6 = \left(5\right)a + b$
$t\left(9\right) = 18 = \left(9\right)a + b$

$a = 3$ and $b = −9$

Now use the sequence to find the rest of the terms of the series.

$\left(−6\right) + \left(−3\right) + 0 + 3 + 6 + 9 + 12 + 15 + 18$