You took out only a numerical factor 
, when you
should have factored out an 
as well. That is,
you should have factored out 
, not just
The complete factorization is:
What you did was not mathematically incorrect, it just was not
complete because the expression inside the parentheses, in
red below, can also be factored:
You can factor out an 
from inside the parentheses,
like this:
and then take away the black parentheses:
and so you end up with the right answer. But it's better
to take out to begin with. Then it's only
one step.
Edwin
(