(i) Let $p:$ the number $x$ is not a rational number
The negation of $p$ is "the number $x$ is a rational number"
This is same as the second statement as if a number is not an irrational number then it is a rational number
Therefore the given statements are negations of each other

(ii) Let $p:$ the number $x$ is a rational number
The negation of the first statement is "the number $x$ is not a rational number" .
This means that the number $x$ is an irrational number which is the same as the second statement
Therefore the given statements are negations of each other