(-x³ +3x +4) -(2x³ +7x² -9) = -x³ +3x +4 -2x³ -7x² +9 . . . . eliminate parentheses
= x³(-1-2) +x²(-7) +x(3) +(4+9)
= -3x³ -7x² +3x +13 . . . matches the second choice.
