设t=3x+1
x=(t-1)/3
f(t)=3×[(t-1)/3]的2次方-(t-1)/3+1
=1/3(t²-2t+1)-1/3t+1/3+1
=1/3t²-2/3t+1/3-1/3t+1/3+1
=1/3t²-t+5/3
所以
f(x)=1/3x²-x+5/3
如果本题有什么不明白可以追问,
另外发并点击我的头像向我求助,请谅解,