Let common difference of series
a₁ , a₂ , a₃ ,..., a_n be d.

$$ \because $$ a₄₀ = a₁ + 39d == –159 ...(i)

and a₁₀₀ = a₁ + 99d = –399 ...(ii)

From eqn. (ii) and (i)
d = –4 and a₁ = –3.

The common difference of the A.P.
b₁, b₂, … , b_m is 2 more than the common
difference of A.P. a₁, a₂, …, a_n.

$$ \therefore $$ Common difference of b₁ , b₂ , b₃ , ..., be (–2).

$$ \because $$ b₁₀₀ = a₇₀

$$ \therefore $$ b₁ + 99(–2) = (–3) + 69(–4)

$$ \therefore $$ b₁ = 198 – 279

$$ \therefore $$ b₁ = – 81