If the first member of the homologous series is methane find the molecular formula of the $2^{nd}$ and the $3^{rd}$ members of the series.

Methane is a member of the alkane series. Alkanes are a group of compounds having a single bond. The general formula is$C_n{H}_{(2n+2)}$. The second and third members have the formulas $C_2{H}_6$ and$C_4{H}_8$. Ethane is the first member of the alkane group$C_2{H}_6$, homologous series having a difference$-C{H}_2$.
Hence, ethane $C_2{H}_6$ is $2^{nd}$ a member, and propane $C_3{H}_8$ is $3^{rd}$ a member.