x^2+2y^2-z^2+3xy-yz
=x²+3xy+9/4y²-y²/4-z²-yz
=(x+3y/2)²-(z+y/2)²
=(x+3y/2-z-y/2)(x+3y/2+z+y/2)
=(x+y-z)(x+2y+z)

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