Answer
$n=12$
Work Step by Step
$_{n}P_k=\frac{n!}{(n-k)!}$
Hence here $_{n}P_6=\frac{n!}{(n-6)!}=12_{n-1}P_5=12\frac{(n-1)!}{((n-1)-5)!}\\\frac{n!}{(n-6)!}=12\frac{(n-1)!}{(n-6)!}\\n(n-1)(n-2)(n-3)(n-4)(n-5)=12(n-1)(n-2)(n-3)(n-4)(n-5)\\n=12$