From the given table, the points can be written as : (6,7.55) , (12,7.85), (18, 8.15) and (24, 8.45)
Slope 
m = 
m = 0.05
slope intercept form of line = 
We plug m = 0.05, x = 6 and y = 7.55 and find c.

c= 7.25
Required equation:
... Answer
The sum is 12x3+18x2-10x+8
I did click on the picture but I can't see the problems it is zoomed in.