In an arithmetic sequence, each term differs by a common difference $d$.

The 8th term minus the 7th term gives $d$:

$d = 12 - 4 = 8$

Working backwards from the 7th term to the 2nd term (going back $5$ steps):

$a_2 = a_7 - 5d = 4 - 5(8) = 4 - 40 = -36$