<span>5 ,1 ,-3,-7 ? what the dickens is going on?
notice, from 5 to 1, is really -4 units,
from 1 to -3, is -4 units as well, 1 -4 = -3 and so on.
so, to get the next term's value, you simply subtract 4 from the current one, namely -4 is the "common difference".
and we also know the first term is 5, ok... then
</span>

<span>
</span>