How to get second line

dear student

p l e a s e s e e t h e s t e p s \frac{(n - r) \times (n - 1)!}{(n - r)!} + \frac{r \times (n - 1)!}{(n - r)!} w e t a k e t h e c o m m o n f a c t o r \frac{(n - 1)!}{(n - r)!} f r o m b o t h t e r m s = \frac{(n - 1)!}{(n - r)!} ((n - r) + r) = \frac{n (n - 1)!}{(n - r)!}

regards