In △PQR, the side lengths (in cm) are given by PQ=3p+4, QR=4p−2 and PR=5p, where p is a positive real number.
a. (a) Write the condition that must be satisfied by the three given expressions so that they represent the sides of a triangle.
b. (b) Determine the value of p for which △PQR is right-angled.
c. (c) Identify the vertex of the right angle in △PQR for the value of p obtained in part (b), and justify your answer.