a, `(x+y-z)³-x³-y³+z³`
=`[(x+y)-z]³-x³-y³+z³`
=`(x+y)³-3(x+y)²z+3(x+y)z² - z³-x³-y³+z³`
=`[x³+y³-z³+3(x+y)(x+z)(y+z)]-x³-y³+z³`
=`3(x+y)(x-z)(y-z)`
b, `(x-y+z)³-x³+y³-z³`
=`[(x-y)+z]³-x³+y³-z³`
=`(x-y)³-3(x-y)²z+3(x-y)z² +z³ -x³+y³-z³`
=`[x³+y³-z³+3(y-z)(x+z)(y-z)]-x³-y³+z³`
=`3(y-z)(x+z)(y-z)`