令3x-1=t,则x=(t+1)/3,于是
f(t)=[(t+1)/3]²+2*(t+1)/3+4
=(t²+2t+1)/9+(2t+2)/3+4
=(t²+2t+1+6t+6+36)/9
=(t²+8t+43)/9
所以
f(x)=(x²+8x+43)/9