(x-2)^2 - 9(x+1)^2 = 0
(x-2)^2 - [3(x+1)]^2 = 0
由平方差公式得：[(x-2) + 3(x+1)][(x-2) - 3(x+1)] = 0
整理得：(4x + 1)(-2x - 3) = 0
所以x = -1/4 或 -3/2