已知数列 {an}\{a_n\}{an} 满足 a1=2a_1 = 2a1=2,且对任意正整数 nnn,有 an+1−an=3n−1a_{n+1} - a_n = 3n - 1an+1−an=3n−1。则 a6a_6a6 的值为( )