Answer:
an = -39 -100(n -1)
Step-by-step explanation:
The given sequence can be described by a 3rd degree polynomial.* However, we suspect a typo, and that your intention is to have a formula for the arithmetic sequence ...
-39, -139, -239, -339
This has a first term a1 = -39, and a common difference d = -100.
The model for the explicit formula is ...
an = a1 +d(n -1)
Filling in the given values, the formula you seek is ...
an = -39 -100(n -1)
_____
* That polynomial is ...
an = 50n^3 +350n^2 -800n +461
This gives a sequence that starts ...
-39, -139, -139, -339, -1039, -2539, -5139, -9139, ...
