Answer:
The third side has a length of 4x² + x - 16
Step-by-step explanation:
We can solve this by taking the value given for the perimeter, and subtracting the values of the other two sides:
3x² + 8x - 9 - (x² + 11x + 5) - (-2x² - 4x + 2)
= 3x² + 8x - 9 - x² - 11x - 5 + 2x² + 4x - 2
= (3 - 1 + 2)x² + (8 - 11 + 4)x - (9 + 5 + 2)
= 4x² + x - 16
