The question is asking for the probability of the event ( or , or both.)
Refer to the diagram attached. There are three mutually-exclusive ways to satisfy :
is satisfied but isn't.
and are both satisfied.
is satisfied but isn't.
Probability that is satisfied but isn't:
.
Probability that and are both satisfied:
.
Probability that is satisfied but isn't:
.
There's no intersection between these three ways for satisfying . Hence, the probability would be the sum of the probability of each of the three ways:
.
Instead of calculating and separately, the work above may be condensed into one equation: