The answer is (1, 2), or 1

Classification discussion according to the idea of program block diagram:

1 If x ∈ [- 1, 1], the output function is f (x) = 3 x

According to the known conditions, 1/9

Because 3 a is a monotonically increasing function, there is -2.

2 If x∈(-∞,-1)∩( 1, ∞), the output function is f (x) = 3 (-x).

According to the known conditions, 1/9

Because 3 a is a monotonically decreasing function, there is -2.

So 1