2.f(x- 1)= x2 =(x- 1)2+2(x- 1)+ 1，f (x) = x 2+2x+ 1。

3. Let f(x)=ax+b, then f (f (x)) = f (ax+b) = a (ax+b)+b = A2x+AB+B = 9x+4.

Then, A 2 = 9, ab+b = 4, and a=3, b= 1, or a=-3, b=-2.

F(x)=3x+ 1 or f(x)=-3x-2.