OG详解-OG7 数学2 Q18

Choice D is correct. Adding the second equation to the first equation in the given system of equations yields 3y-3y = 4x+9x+17-23, or 0 = 13x-6. Adding 6 to each side of this equation yields 6 = 13x. Multiplying each side of this equation by 3 yields 18 = 39x. Therefore, the value of 39x is 18.
Choice A is incorrect. This is the value of -39x, not 39x. Choice B is incorrect. This is the value of -13x, not 39x. Choice C is incorrect. This is the value of 13x, not 39x.