take u^2=t
so eqn reduces to
t^2-t-6
now 6 has factorisation as=2*3*1
middle term=-1*t
so u want to get -t
this u can get by splitting it into -3t+2t
now eqn is t^2-3t+2t-6
t(t-3)+2(t-3)
(t+2)(t-3)
substitute value of t back u get
(u^2+2)(u^2-3)
take u^2=t
so eqn reduces to
t^2-t-6
now 6 has factorisation as=2*3*1
middle term=-1*t
so u want to get -t
this u can get by splitting it into -3t+2t
now eqn is t^2-3t+2t-6
t(t-3)+2(t-3)
(t+2)(t-3)
substitute value of t back u get
(u^2+2)(u^2-3)