Answer:
Step-by-step explanation:
You want to group things that can actually be factored. You dont want to group -7x^3 and -9 because that would just be a mess and wouldnt give you the right answer.
[tex](-7x^3+21x^2)+(3x-9)\\-7x^2(x-3)+3(x-3)\\(-7x^2+3)(x-3)[/tex]
