OG详解-OG8 数学1 Q4

Choice D is correct. The given expression is equivalent  to (2x² +x +(-9))+(x² +6x +1), which can be rewritten as (2x² +x²)+(x+6x)+(-9+1). Adding like terms in this expression yields 3x² +7x +(-8), or 3x² +7x-8.
Choice A is incorrect and may result from conceptual or calculation errors. Choice B is incorrect and may result from conceptual or calculation errors. Choice C is incorrect and may result from conceptual or calculation errors.