This exercise is from Introduction to Modern Cryptography (2nd Edition) by Katz & Lindell

DEFINITION 3.8 A private-key encryption scheme = (Gen, Enc. Dec) has indistinguishable encryptions in the presence of an eavesdropper, or is EAV secure, if for all probabilistic polynomial-time adversaries A there is a negli- gible function negl such that, for all n, Privkm()-negn) where the probability is taken over the randomness used by A and the ran- domness used in the ezperiment (for choosing the key and the bit b, as well as any randomness used by Enc)