Factorise

$(x+1)^{3}+(x-1)^{3}$

$=(x+1+x-1)\left[(x+1)^{2}+(x-1)^{2}-(x-1)(x+1)\right]$

$=(2 x)\left[(x+1)^{2}+(x-1)^{2}-\left(x^{2}-1\right)\right]$

$=2 x\left(x^{2}+1+2 x+x^{2}+1-2 x-x^{2}+1\right)$

$=2 x\left(x^{2}+3\right)$