when you eliminate parenthesis, and the sign to the left of the parenthesis is a "+", just remove the parenthesis, nothing else changes.
but if the sign to the left of the parenthesis is a "-", negative, remove the parenthesis, all signs inside the parenthesis change to the opposite, that is, "+" becomes "-", and a "-" becomes a "+'
in this case, it is a "-" to the left of the second parenthesis, so
-5v³-9v²+6v+5v³-9v²+6v=-18v²+12v