OG详解-OG3 数学2 Q15

Choice C is correct. It’s given by the first equation in the system that y = 3x. Substituting 3x for y in the equation 2x + y = 12 yields 2x +3x = 12, or 5x = 12.
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 D is incorrect and may result from conceptual or calculation errors.