Answer: 8n
==========================================================
Explanation:
The two sides 4n+5 and 5n+6 have the terms 4n and 5n which add to 9n. We need 8n to add onto this so that we end up with 17n. In other words, 4n+5n+8n = 9n+8n = 17n
So that is why the answer is 8n. We don't have anything added or subtracted to it because the "+5" and "+6" in the two given expressions (4n+5 and 5n+6) add up to 5+6 = 11 which is what we want in the perimeter expression 17n+11
Side 1 = 4n+5
Side 2 = 5n+6
Side 3 = 8n
Perimeter = (side1) + (side2) + (side3)
Perimeter = (4n+5) + (5n+6) + (8n)
Perimeter = 4n+5 + 5n+6 + 8n
Perimeter = (4n+5n+8n) + (5+6)
Perimeter = 17n+11
So that confirms we have the proper expression for the third missing side.
