Representing a transmission line by the two-port network, in terms of A B C D A B

Representing a transmission line by the two-port network, in terms of $ABCD$ parameters, (a) express $V_S$, the sending-end voltage, in terms of $V_R$, the receiving-end voltage, and $I_R$, the receiving-end current, and (b) express $I_S$, the sending-end current, in terms of $V_R$ and $I_R$.
(a) $V_S=$ __________
(b) $I_S=$ __________