Rewrite: \( x^2 - 9y^2 = (x - 3y)(x + 3y) = 0 \).
The equation represents two lines:
1. \( x - 3y = 0 \Rightarrow x = 3y \Rightarrow y = \frac{x}{3} \).
2. \( x + 3y = 0 \Rightarrow x = -3y \Rightarrow y = -\frac{x}{3} \).
Answer: \( x - 3y = 0 \), \( x + 3y = 0 \).
