k = 18
the common difference d of an arithmetic sequence is
d = a₂ - a₁ = a₃ - a₂ = ....
a₂ - a₁ = 2k - 1 - k - 9 = k - 10 and
a₃ - a₂ = 2k + 7 - (2k - 1) = 2k + 7 - 2k + 1 = 8, hence
k - 10 = 8 ⇒ k = 8 + 10 = 18
the 3 consecutive terms are
27, 35, 43
