a)$ P(x)= 2$$x^{4}$$+3x³+3x²-$$x^{4}$$-4x+2x²+6x$
= $x^{4}$$+3x³+5x²+6x$
Q(x)=$x^{4}$$+3x²+5x-1-x²-3x+2+x³$
=$x^{4}$$+x³+2x²+6x+1$
b)P+Q=$x^{4}$$+3x³+5x²+6x+$$x^{4}$$+x³+2x²+6x+1$
=$2x^{4}$$+4x³+7x²+12x+1$
P-Q= $x^{4}$$+3x³+5x²+6x-$($x^{4}$$+x³+2x²+6x+1)$
=$x^{4}$$+3x³+5x²+6x-$$x^{4}$$-x³-2x²-6x-1$
$=2x³+3x²-1$