[tex]\displaystyle\bf\\n=5 \ \ \ \Rightarrow \ \ \ a_{n+3} =a_{5+3} =a_{8} \\\\\\a_{8} =?\\\\a_{n+1} =3n^{2}-8 \\\\a_{7+1} =3\cdot 7^{2} -8\\\\a_{8} =3\cdot 49-8\\\\a_{8} =147-8=139\\\\Otvet: \ a_{n+3} =139[/tex]