Answer:
129
Step-by-step explanation:
The easiest thing to do would be an equation.
Since the value is decreasing by 6 each time you have -6x
-6x+?=?
Next thing you do, to get the y-intercept, would be to add 6 to the 309, which is the first term in order to get the zeroth term. This would make 315 the y-intercept
-6x+315=?
To solve for the 31st term, plug in 31 for x to get 129
