The Tribonacci sequence is defined by an=an−1+an−2+an−3a_n = a_{n-1} + a_{n-2} + a_{n-3}an=an−1+an−2+an−3 with a0=0,a1=1,a2=1a_0 = 0, a_1 = 1, a_2 = 1a0=0,a1=1,a2=1. What is a6a_6a6?
13
17
21
24