I assume by P(,3) you meant P(n, 3). I assume that the notation represents permutations. You should know a basic formula which allows the equation to be written as

Now you have three choices:

Option 1:Guess and check. When you get to n = 10 you'll have the answer.

Option 2:By definition of the factorial, the equation can be written as

.

Therefore n = 10 (the negative solution is rejected).

Option 3:Note that . Therefore . This suggests that the left and right hand sides are equal to 7!. Hence n = 10. Check it ..... Correct.