Find a7 in the geometric series in which s5 = -61 and the common ratio is -3

Formula for finding sum of terms in a geometric progression is:

Sn = a1 ( 1 - r^n) / (1 - r)

-61 = a1 (1 - -3^5 ) / ( 1 - - 3

-61 = a1 ( 1 - - 243 ) / 4

a1 = 4/ 244 * -61

a1 (The first term is therefore) = -1

The seventh term, a7, will be given by: an = a1 * r^n-1

a7 = -1 * ( -3 )^7-1

= -1 * (-3^6)

= - 729