Resolução:
2.A(x, 4) = 4!.C(x, x – 5)
2.x!/(x – 4)! = 4!.x!/(x – 5)!.[x – (x – 5)]!
2.x!/(x – 4)! = 4!.x!/(x – 5)!.(x – x + 5)!
2.x!/(x – 4)! = 4!.x!/(x – 5)!.5!
divide tudo por x!:
2/(x – 4)! = 4!/(x – 5)!.5!
2/(x – 4)! = 4!/(x – 5)!.5.4!
2/(x – 4)! = 1/(x – 5)!.5
2/(x – 4).(x – 5)! = 1/(x – 5)!.5
multiplica tudo por (x – 5)!:
2/(x – 4) = 1/5
2.5 = 1.(x – 4)
10 = x – 4
x = 14