Let a = i^ + j^ ​ +k^, b = −i^ − 8j^ ​+ 2k^ and c = 4i^ + c2​j^ ​ +c3​k^ be three vectors such that b × a = c × a. If the angle between the vector c and the vector 3i^ + 4j^ ​ +k^ is θ, then the greatest integer less than or equal to tan²θ is :