Question. If the sum of first $12$ terms of an A.P. is equal to to the sum of the first $18$ terms of the same A.P., find the sum of the first $21$ terms of the same A.P.
$a=$ first term
$d=$ difference
I know now, $2(2a + 11d) = 3(2a + 17d)$
hence, $2a + 29d = 0$ --> correction
How do I proceed from here?