Hi
month totals increase ... by 8 | recorded: 21, 29, 37, 45, and 53
slope is 8
S(m) = 8m + b
FIRST MONTH... S(1)= 8*1 + b
21 = 8 + b
13 = b
S(m) = 8m + 13 |(S) as a function of the number of months (m)
Check Your answer using any of the monthly totals given:
s(4) = 45 as one example