a) N = 15y^3+5y^2-1+y^3-y^3+7y^5
=7y5+15y3-y3+y3+5y2-1
=7y5+15y3+5y2-1
M = y^2+y^3-3y+1-y^2+y^5-y^3+7y^5
=y5+7y5+y3-y3+y2-y2-3y+1
=8y5-3y+1
b) M+N=8y5-3y+1+7y5+15y3+5y2-1
=7y5+8y5+15y3+5y2-3y+1-1
=15y5+15y3+5y2-3y
N-M=7y5+15y3+5y2-1-8y5+3y-1
=7y5-8y5+15y3+5y2+3y-1-1
=-y5+15y3+5y2+3y-2