C(x)+B(x)=A(x)
C(x)+($-x^{5}$ -$3x^{3}$ +$2x^{2}$ +x-15)=$3x^{6}$ -$x^{5}$ -$3x^{3}$ +$x^{2}$ +x-1
C(x)=$3x^{6}$ -$x^{5}$ -$3x^{3}$ +$x^{2}$ +x-1-($-x^{5}$ -$3x^{3}$ +$2x^{2}$ +x-15)
C(x)=$3x^{6}$ -$x^{5}$ -$3x^{3}$ +$x^{2}$ +x-1$+x^{5}$ +$3x^{3}$ -$2x^{2}$ -x+15
C(x)=$3x^{6}$+(-$x^{5}$+$x^{5}$ )+(-$3x^{3}$+$3x^{3}$)+($x^{2}$-$2x^{2}$)+(x-x)+(-1+15)
C(x)=$3x^{6}$-$x^{2}$ +14
Vậy C(x)=$3x^{6}$-$x^{2}$ +14