9^ ( - |2-x| ) - 4 * 3^( -|2-x| ) + a = 0
[ 3^ ( - |2-x| ) ]^2 - 4 * 3^( -|2-x| ) + a = 0
令t=3^( - |2-x| )
t^2 - 4t + a = 0
(t-2)^2 - 4 + a=0
a = 4 - (t-2)^2
∵- |2-x| ≤ 0
∴0＜3^( - |2-x| ≤ 1
即：0＜t ≤ 1
∴-2 ＜ t-2 ≤ -1
4＞ (t-2)^2 ≥ 1
0＜ 4 - (t-2)^2 ≤ 3
即：0 ＜ a ≤ 3