Four statements driven from the original:
converse:
if a pair of angles are congruent, then the angles are vertical.
inverse:
if a pair of angles are not vertical, then the angles are not congruent.
contrapositive:
if a pair of angles are not congruent, then the angles are not vertical.
conditional:
if a pair of angles are vertical, then the angles are congruent.
b)
If you know that a statement is true, then its contrapositive form must be true as well.