Răspuns:
n!/3! •(n-3)!=2n. "!" = factoriale
(n-3)! • (n-2) •(n-1) •n/ 6•(n-3)! =2n
simplificare (n-3)!
(n-2)(n-1)n/6=2n | :n
n²-3n+2/6=2
n²-3n+2=12
n²-3n-10=0
delta= 9-4•1• (-10) = 49
n= 3+7/2=5
n= 3-7/2= -2