Final answer:
The missing fence length can be determined by subtracting the total length of the given sides from the total length of the fence around the quadrilateral-shaped garden.
Step-by-step explanation:
The missing fence length can be determined by subtracting the total length of the given sides from the total length of the fence around the quadrilateral-shaped garden.
Expression: (3a^2 + 15a + 9) - (A + B + C)
Example: If the total length of the given sides A, B, and C is 4a^2 + 8a + 6, then the expression for the missing fence length will be (3a^2 + 15a + 9) - (4a^2 + 8a + 6).