3a³-a²+2a-4=(3a³-3)-(a²-2a+1)=3(a³-1)-(a-1)²=3(a-1)(a²+a+1)-(a-1)²=3(a-1)(a²+a+1-a/3+1/3)=3(a-1)(a²+2a/3+4/3)=(a-1)(3a²+2a+4)