P(x)+Q(x)= ( 5x2+2y3+7x5 y6 -1/2xy)+(3x-9x5y6+1/2x2y-5xy)
= 5x2+2y3+7x5 y6 -1/2xy+3x-9x5y6+1/2x2y-5xy
= 5x2+2y3+(7x5 y6-9x5y6)+(-1/2xy-5xy)+3x+1/2x2y
=5x2+2y3-2x5 y6-11/2xy+3x+1/2x2y
P(x)-Q(x)=(5x2+2y3+7x5 y6 -1/2xy)-(3x-9x5y6+1/2x2y-5xy)
= 5x2+2y3+7x5 y6 -1/2xy-3x+9x5y6 - 1/2x2y+5xy
=5x2+2y3 +(7x5 y6 +9x5y6)+(-1/2xy +5xy)-3x - 1/2x2y
= 5x2+2y3 +16x5 y6 +9/2xy -3x - 1/2x2y