Question 847520
sum from i =1 to n  of (15-2i) = 45

sum from i = 1 to n of 15  - 2* sum from i = 1 to n of i

15n - n(n+1) = 45

15n - n^2 + n = 45

n^2 -14n +45 = 0

(n-5)(n-9)

n=5,  n=9

Check:  13+11+9+7+5 = 45 

Check:  13+11+9+7+5+3+1+-1+-3 = 45

So 5 terms or 9 terms.