Assume ABC and PQR to be 2 triangle.

We, have

ΔABC ~ΔPQR

Perimeter of ΔABC = 25cm

Perimeter of ΔPQR = 15cm

AB = 9cm

PQ = ?

Since, ΔABC ~ΔPQR

Then, ratio of perimeter of triangles = ratio of corresponding sides

So, (Corresponding parts of similar triangle area proportion)

Or

Or PQ = 135/25

Or PQ = 5.4 cm