= Solution
Read the stronger members of the inset doublets at about $2945$, $2926$ and $2906\ \mathrm{cm^{-1}}$. Their spacings are roughly $19$–$20\ \mathrm{cm^{-1}}$, so the common-$B$ estimate is
$$
\boxed{B\simeq\tfrac12\Delta\widetilde\nu\simeq10\ \mathrm{cm^{-1}}.}
$$
Using the harmonic wavenumber from the preceding calculation and labelling lines by the lower-state $J$,
$$
\boxed{\widetilde\nu_{P_1}\simeq2886.89-2(10)=2867\ \mathrm{cm^{-1}},\qquad
\widetilde\nu_{R_1}\simeq2886.89+4(10)=2927\ \mathrm{cm^{-1}}.}
$$
These are graph-based estimates. Finite reading precision and different upper/lower <rotational constants> limit their accuracy to a few cm−1; extra decimal places would not be justified. The R1 result refers to $J=1\to2$, while R0 is the line nearest the high-wavenumber side of the band gap.
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