Answer:
[tex]\large\boxed{\left\{\begin{array}{ccc}a_1=3\\\\a_n=2a_{n-1}\end{array}\right}[/tex]
Step-by-step explanation:
[tex]a_1=3\\\\a_2=6=3\cdot2\to a_2=2a_1\\\\a_3=12=6\cdot2\to a_3=2a_2\\\\a_4=24=12\cdot2\to a_4=2a_3\\\vdots\\a_n=2a_{n-1}[/tex]