代入-1得： 1-p+q = -1
代入3得：9+3p+q = 3
解方程：p = -1,q = -3
再看B：f(f(x)) = x
=> (x^2-x -3)^2 -(x^2-x -3) -3=x
=>(x^2-x -3)^2 = x^2
=>x^2-x -3 = x 或x^2-x -3 = -x
解方程就可以了