f(-1) = -2,f(3) = 8a+6,f(2) = 3a+4,f(1) = 2
∵f(-1) < f(3) < f(2) < f(1)
∴有-2 < 8a+6 < 3a+4 < 2
解不等式得 -1 < a < -2/3