Consider this option:
1. this arithmetic sequence consists of b1;b2;b3;b4;...;bₐ, where b1=13.1; b2=3.1; b3=-6.9; b4=-16.9, etc. and the difference (d) between bₐ₊₁ and bₐ is (-10).
2. using formula for bₐ of arithmetic sequence: bₐ=b₁+d(a-1), where a - number of term of this sequence, it is possible to find b₂₉: b₂₉=b₁+d(29-1)=13.1-10*28=-266.9.
answer: -266.9