Answer:
-5x³ + 3x³ + 2x + 6
Explanation:
if that is truly the complete problem description, then the only possible answer I can think of, that makes any sense :
what do I need to add to 5x³+3x²-2x+1 to get 6x²+7 ?
the answer is
6x²+7 - (5x³+3x²-2x+1) = 6x²+7-5x³-3x²+2x-1 =
= -5x³ + 3x³ + 2x + 6