Describe briefly the isotopic substitution in microwave spectrum

From the pure rotational spectra of ${?}^{12}{C}^{16}O$ and ${?}^{13}{C}^{16}O,$ the values of rotational constant for the two have been found to be $1\mathrm{.}92118c{m}^{-1}$ and $1\mathrm{.}83669c{m}^{-1}$ respectively. Calculate the atomic mass of ${?}^{13}C,$ if the atomic masses of ${?}^{12}C$ and ${?}^{16}O$ are $12a$ and $15\mathrm{.}994a$ respectively.