$(1$ point$)$ Alice and Bob are using the ElGamal cipher with the parameters $p=191$ and $=19.$

Alice makes the mistake of using the same ephemeral key for two plaintexts, $x_1$ and $x_2.$ The eavesdropper Eve suspects that $x_1=187.$ She sees the two ciphertexts $y_1=47$ and $y_2=18$ in transit; these are the encryptions of $x_1$ and $x_2,$ respectively.

a$)$ What is the masking key $k_M?$

b$)$ What is the plaintext $x_2?$