a
p(x)=-2x^3+5x^2-5x+3x^3+4/3
=(-2x^3+3x^3)+5x^2-5x+4/3
=x^3+5x^2-5x+4/3
b
p(x)+Q(x)=(x^3+5x^2-5x+4/3)+(x^3+4x^2-2x+1/3)
=x^3+5x^2-5x+4/3+x^3+4x^2-2x+1/3
=(x^3+x^3)+(5x^2+4x^2)+(-5x-2x)+(4/3+1/3)
=2x^3+9x^2-7x+5/3
P(x)-Q(x)=(x^3+5x^2-5x+4/3)-(x^3+4x^2-2x+1/3)
=x^3+5x^2-5x+4/3-x^3-4x^2+2x-1/3
=(x^3-x^3)+(5x^2-4x^2)+(-5x+2x)+(4/3-1/3)
=x^2-3x+1