\(\Leftrightarrow\left[\left(x+y+z\right)^3-x^3\right]-\left(y^3+z^3\right)\)
\(\Leftrightarrow\left[\left(y+z\right)+x\left(x+y+z\right)+x^2\right]-\left(y+z\right)\left(y^2-yz+z^2\right)\)
\(\left(y+z\right)\left(3a^2+3ab+3bc+3ca\right)=3\left(a+b\right)\left(b+c\right)\left(c+a\right)\)