Answer:

Explanation:
Relevant data provided as per the requirement of question is shown below:-
M(c,d)is a midpoint of PQ
PM = MQ
The expression for PM to find an expression for PQ is shown below:-
Where,

Which is equals to MQ
and

For more clarification, the diagram has been presented below:-
I______I______I
P M Q
(a,b) (c,d)