Final answer:
Without additional information about parallelogram PQRS, we cannot determine the value of segment PT in terms of 'a'.
Step-by-step explanation:
The value of segment PT in terms of 'a' in parallelogram PQRS with diagonals PR and QS intersecting at point T cannot be determined from the provided information. The question seems to provide details about vectors and charges, which are related to physics, and does not give any specific information about the lengths of the diagonals or sides of the parallelogram in terms of 'a'. Consequently, without additional geometrical information pertaining to the parallelogram or the relationship between 'a' and the lengths of the segments, we are not able to match the element to its value.