A bag holds 7 red, 5 green and 3 white tokens of the same size…

The complement of $E$ is $\text{not } E$. Every outcome favours exactly one of them, so $P(E) + P(\text{not } E) = 1$ and $P(\text{not } E) = 1 - P(E)$. When an event is hard to count, count its complement.