Answer:
Side 3 is: 5x^3 - 4x^2 +3x - 12
Note that the answer options do not include this result, but it is likely that the second option is simply missing the square term for the -4x, and it should read -4x^2.
Explanation:
The perimeter is the sum of all three sides. To find Side 3, subtract Sides 1 and 2 from the perimeter:
5x^3 - 2x^2 +3x - 8 - (3x^2-4x-1) - (4x-x^2+5) = Side 3
Perimeter - Side 1 - Side 2 = Side 3
5x^3 - 2x^2 +3x - 8 - 3x^2+4x+1 - 4x+x^2-5 [Remove parentheses]
5x^3 - 3x^2 +x^2 - 2x^2 +3x +4x - 4x - 8 +1 -5 [Combine like terms]
5x^3 - 4x^2 +3x - 12 [Simplify] = Side 3