\(a) M(x)+N(x)=5x^3-10x^2-8x+10+3x^3+6x^2-3x+7\\ =(5x^3+3x^3)+(-10x^2+6x^2)+(-8x-3x)+(10+7)\\ =8x^3-4x^2-11x+17\\\\ b) M(x)-N(x)=(5x^3-10x^2-8x+10)-(3x^3+6x^2-3x+7)\\ =5x^3-10x^2-8x+10-3x^3-6x^2+3x-7\\ =(5x^3-3x^3)+(-10x^2-6x^2)+(-8x+3x)+(10-7)\\ =2x^3-16x^2-5x+3\)