One way to do this problem is to determine the common difference. If the 5th and 8th terms are -9 and -21, we can do this by subtracting -9 from -21:
-21-(-9) = -12. The 5th and 8th terms are not consecutive, so we have to think in terms of (8-5), or 3, times the common difference to get from -9 to -21.
Note that -12 divided by 3 is -4. Thus, the common difference is -4.
We know that the 5th term is -9. To find the 4th term, work backwards: subtract (-4) from -9, which produces -9+4=-5.
The fourth term is -5. Subtracting -4 from this (which is the same as adding 4 to this -5) produces the third term; it is -1. Can you now find the 2nd and 1st terms using the same approach?
