-1 - (-7) = 6
5 - (-1) = 6
11 - 5 = 6
The common difference is 6.
If you add 6 to a term, you get the next term.

DONT OPEN THE ANSWER ABOVE ITS A VIRUS. i will give an answer in the comments though
The 3 that represent in this function is the "A. The amount of rainfall in week 0." zero weeks have passed, and there's already up to the 3 mark. if additional rain comes into the bin, the existing water is still there<span>so we add +3 every time</span>