Помогите с алгеброй пж-ста!! S3=28; q=1/2; S7-?

Sn= b1(1-q)^1-n /1-q S3=b1(1-1/2)^2/1-1/2 => 28=b1*(1-1/2)^2/1-1/2 => b1=28*1/2/0,25=56 S7=b1(1-q)^6/1-q =>S7=56(1-1/2)^6/1-1/2=0,175