Question 726671
{{{3(x+1)^2-9(x+1)-12}}}

let (x+1)=a

{{{3a^2-9a-12}}}


{{{3a^2-12a+3a-12}}}

{{{3a(a-4)+3(a-4)}}}

{{{(a-4)(3a+3)}}}

{{{3(a-4)(a+3)}}}

re substitute
{{{3(x+1-4)(x+1+3)}}}

{{{3(x-3)(x+4)}}}