Part A
[tex]3x^2 -2x-1+9x+2x^2 -3=\boxed{5x^2 + 7x-4}[/tex]
Part B
[tex](5x^3 +4x^2 - x-3)-(5x^2 + 7x-4)=\boxed{5x^3 - x^2 - 8x+1}[/tex]
Part C
No, because although they provide examples for specific polynomials being closed under addition and subtraction, there is a loss of generality.