The length of two sides of a triangle are shown below:
Side 1: 8x^2 - 5x - 2
Side 2: 7x - x^2 + 3
The perimeter of the triangle is
4x^3 - 3x^2 + 2x - 6.

Part a: what is the total length of the two sides, 1 and 2, of the triangle?

Part b: what is the length of the third side of the triangle?

Part c: are the answers for part a and part b an example demonstrating that polynomials are closed under addition and subtraction? Justify your answer.