How many terms of the arithmetic sequence 7,9,11.....should be added to get 391

apply the formula

Sn =n/2(2a +(n_1)d )

391 =n/2 (2*7+(n-1)2)
391=n/2 (14+2n_2)
391=n/2 *2(7+n_1)
391=n(6+n)
391=6n +n^2
n^2+6n_391 =0
n^2+23n_17n_391 =0
n(n+23) _17(n+23)
(n_17) (n+23)
n=17
n=_23
there fore we neglect the negative value
ans =17