So you have this function,
[tex]\large\rm f(x)=(x+1)(x^2-9)[/tex]
Maybe ignore the x in the function for now.
Just think of it as an empty square that you fill in with something,
[tex]\large\rm f(\square)=(\square+1)(\square^2-9)[/tex]
And they're asking us to simplify the function when the square is filled with x-1,
[tex]\large\rm f(x-1)=([x-1]+1)([x-1]^2-9)[/tex]
You'll need to expand the (x-1)^2 term to (x^2-2x+1),
[tex]\large\rm f(x-1)=([x-1]+1)([x^2-2x+1]-9)[/tex]
[tex]\large\rm f(x-1)=(x-1+1)(x^2-2x+1-9)[/tex]
And then simplify further,
[tex]\large\rm f(x-1)=(x)(x^2-2x-8)[/tex]
And distribute the x,
[tex]\large\rm f(x-1)=x^3-2x^2-8x[/tex]
