a, M+N= x^2-2yx+z^2+3yz-z^2+5x^2
= (x^2+5x^2)+(-2yx)+(z^2-z^2)+3yz
=6x^2+-2yx+2z^2+3yz
b, M-N=(x^2-2yx+z^2)-(3yz-z^2+5x^2)
= x^2-2yx+z^2-3yz+z^2-5x^2
=(x^2-5x^2)+(-2yx)+(z^2+z^2)+(-3yz)
=-4x^2+-2yx+2z^2+-3yz
N-M=(3yz-z^2+5x^2)-(x^2-2yx+z^2)
= 3yz-z^2+5x^2-x^2+2yx-z^2
=3yz+(-z^2-z^2)+(5x^2-x^2)+2yx
=3yz+6x^2+2yx