P(E’) is

Notation P(E’) signifies the probability when the event E must not occur.

The Probability that E will occur is P(E), and since only 2 outcomes are possible i.e., E will occur, and E will not occur.

∴ P(E) + P(E’) = 1 (∵ Total probability is 1)

⇒ P(E’) = 1 – P(E)