13P5 = 1287(xPx)
I simplify the above to: 13!/8! = 1287(x!)
The expression on the left simplifies to 154,440.
I divide both sides by 1287 to get: 120=x!
At this point I'm stumped.
Thanks in advance, n
I am not sure that there is really a better solution than to just calculate the first few factorials at this point and see which one gives you $120$... You should find your answer pretty fast.
It is not possible to invert the "factorial" function in general since it is not injective, i.e. $0!=1!=1$