\(P\left(x\right)+Q\left(x\right)-R\left(x\right)\)
\(=-x^3-\dfrac{1}{2}x^2+x+10-4x^3-2x+1\)
\(=-5x^3-\dfrac{1}{2}x^2-x+11\)
\(P\left(x\right)-Q\left(x\right)+R\left(x\right)\)
\(=-2x^3+\dfrac{1}{4}x^2+5+\dfrac{3}{4}x^2-x^3-5-x+R\left(x\right)\)
\(=-3x^3+x^2-x+4x^3-2x+1=x^3+x^2-3x+1\)