The lengths of two sides of a triangle are shown. Side 1: 3x^2 - 4x - 1
Side 2: 4x - x^2 + 5

The perimeter of the triangle is 5x^3 - 2x^2 + 3x - 8.

Part A: What is the total length of the two sides, 1 and 2, of the triangle? Show your work.

Part B: What is the length of the third side of the triangle? Show your work.

Part C: Do the answers for Part A and Part B show that the polynomials are closed under addition and subtraction? Justify your answer.