Choice A is the correct answer. Coincident lines.

1. Compare: Equation 2 should be a multiple of Equation 1.
2. Ratio: $6x / 3x = 2$. The multiplier is 2.
3. Apply: Multiply the y-term of Eq 1 by 2: $(-1) \times 2 = -2$.
4. Match: The y-term in Eq 2 is $-k$. So $-k = -2 \rightarrow k = 2$.

💡 Strategic Tip: For infinite solutions, $A_1/A_2 = B_1/B_2 = C_1/C_2$.

Choice B is incorrect because sign error. Choice C is incorrect.Choice D is incorrect.