Binomial expansion of (X+Y+Z)^4

eq1: (x+y+z)^(4)=x^(4)+y^(4)+z^(4)+4[x^(3)y+x^(3)z+y^(3)x+y^(3)z+z^(3)x+z^(3)y]+6[x^(2)y^(2)+x^(2)z^(2)+z^(2)y^(2)]+12[x^(2)yz+y^(2)xz+z^(2)xy]

Concept of binomial expansion