from introduction to modern cryptography (Kats and Lindell, 2nd edition)

11.10 In Section 11.4.4 we showed that El Gamal encryption is malleable, and specifically that given a ciphertext (c, c2) that is the encryption of some unknown message m, it is possible to produce a ciphertext (ci, c2) that is the encryption of a m (for known a). A receiver who receives both these ciphertexts might be suspicious since both ciphertexts share the first component. Show that it is possible to generate ,C) that is the encryption of a m, with c / c and 2 c2