The given statements are

1." If a quadrilateral is a parallelogram then its diagonals bisect each other."

2. "If the diagonals of a quadrilateral do not bisect then it is not a parallelogram"

Here the second statement is contra positive of first one.

Because for first statement,

$p\rightarrow$ The quadrilateral is a parallelogram.

$q\rightarrow$ The diagonals of a quadrilateral bisect.

~$p\rightarrow$ The quadrilateral is not a parallelogram.

~$q\rightarrow$ The diagonals of a quadrilateral do not bisect.

The contra positive of ' If $p$ then $q$ is If ~$q$ then ~$p$