i love it when i can surprise myself.... your first mistake is by starting with a negative number (-3x^3 should be +3x^3) your second mistake is with the 7x and the 9x (4 and 5 rows up from your last line) since they're both negative numbers your the equation reads: -7x - -9x or -7x+9x which would equal +2x. let me know if i can explain it better and i'll try