由题有：f(3X+1)=2X^2-3X,令t=3X+1,则有X=(t-1)/3,所以有 f(3X+1)=f(t)=2X^2-3X=2[(t-1)/3]^2-3[(t-1)/3],化简后得,f(t)=(2t^2-13t+11)/9,即所求f(x)=(2X^2-13X+11)/9=(X-1)(2X-11)/9