设正方形边长为x厘米.
(x + 6)(x - 4) = x²
x² + 6x - 4x - 24 = x²
2x = 24
x = 24 ÷ 2
x = 12
12 × 12 = 144平方厘米
答：原先的正方形面积是144平方厘米.