x^2-2x-3≤0
(x+1)(x-3)≤0
-1≤x≤3
y=2^(x+2)-2^(2x+1)
=4*2^x-4*2^2x
设2^x=t∈[1/2,8]
y=-4t^2+4t
=-4(t-1/2)^2+1
当t=1/2时,最小值：1