Choice C is the correct answer. To verify if $(6, 3)$ is a solution, we substitute $x = 6$ and $y = 3$ into both equations.

First equation: $x - 2y = 0$$6 - 2(3) = 6 - 6 = 0$ ✓ (True)

Second equation: $3x + y = 21$$3(6) + 3 = 18 + 3 = 21$ ✓ (True)

Since the point satisfies both equations, it is a solution to the system.

💡 Strategic Tip: A point is only a solution if it works for ALL equations in the system.

Choice A is incorrect because the point satisfies both equations, not just the first one.

Choice B is incorrect because we can always determine if a point is a solution through substitution.

Choice D is incorrect because the point does satisfy both equations.