81a²-25 would be the answer I'm pretty sure. Hopefully I helped.
Ok so for number 5, AB is proportional to BC just like PQ is proportional to QR. AB = 9, BC = 15, PQ = 3, and QR = x. 9/15 = 3/x. Cross Multiply: This gives you - 9x = 45. Divide both sides by 9, and you get x = 5. QR = 5
Ok what is the question lol
Found this when I was doing my own work, hope this helps.