\(P\left(x\right)=-2x^3+3x^2+4x+6\)
\(Q\left(x\right)=-2x^3+2x^2+4x+15\)
\(P\left(x\right)+Q\left(x\right)=-4x^3+5x^2+8x+21\)
\(P\left(x\right)=Q\left(x\right)\)
\(\Leftrightarrow-2x^3+3x^2+4x+6=-2x^3+2x^2+4x+15\)
\(\Leftrightarrow x^2=9\)
\(\Leftrightarrow x=\pm3\)