请问 a1=3,a2=20,a3=119,an=6an−1−an−2+2a_1=3,a_2=20,a_3=119,a_n= 6a_{n-1}-a_{n-2}+2a1=3,a2=20,a3=119,an=6an−1−an−2+2 的第 aia_iai 项的通项公式怎么求
