Arithmetic: (3,7,11,\ldots) → (a_n=3+(n-1)4=4n-1)

Geometric: (2,6,18,\ldots) → (b_n=2\cdot 3^{n-1})

[ t_n = (4n-1)\cdot 2\cdot 3^{n-1} = 2(4n-1)3^{n-1} ]

Answer: (t_n=2(4n-1)3^{n-1})