设小正方形的边长是 x 厘米,则大正方形的边长是 x + 3 厘米
4(x + 3) = 2×4x
4x + 12 = 8x
8x - 4x = 12
4x = 12
x = 12÷4
x = 3
答:小正方形的边长是 3 厘米.