If e and f are not disjoint events, then p(e or f)equals

The P(e or f) is the P(e) + P(f) when they are disjoint events.

Disjoint events are events that are independent of each other.

Imagine rolling a die. The probability of rolling a 1 is independent of rolling a 2. Both of these probabilities are 1/6. 

To find the probability of rolling either a 1 or 2, P(1 or 2) we simply add the 2 probabilities together.

X> -2 is the simplified version.