If the probability of something happening is x, then the probability of it not happening is 1 - x.
Example
If the probability that it will rain is 3/8, then the probability that it will not rain is 1 - 3/8 = 5/8
Example
The probability of the spinner landing on yellow is 3/8.
The probability of the spinner not landing on yellow is 1 - 3/8 = 5/8