Answer:

Explanation:
First, it's never a bad idea to draw the line and see what it looks like! This is shown below (not to scale).
We are given that PS = 18 and PR = 15, and we want to determine QR.
PS is the sum of PR and RS:

Substitute:

Solve for RS

Since RS ≅ PQ:

PS is also the sum of PQ, QR, and RS. Hence:

Since PS = 18 and RS = PQ = 3:

Solve for QR:

In conclusion, QR measures 12 units.