Part a

tmp = 
      Expand[(1 - t1 B - t2 (B^2)) (1 - T (B^12))]

tmp2 = Expand[(tmp^2)]

f[p_] := Coefficient[tmp2, (B^p)]
Map[f, Range[1, 28]]

Part b

tmp = 
      Expand[(((1 - t1 B - t2 (B^2) - t12 (B^12)))^2)]

f[p_] := Coefficient[tmp, (B^p)]
Map[f, Range[1, 28]]