As you have 4 digist to create as many 4 digit numbers as you can you just have to think, how many starters do I have?
You have:
7
6
5
3
Those are how many you can start your number with, and then how many options of arrangements do I have for each one? and there are just three number to arrange in three different ways after that, it means that you have 4x3 ways to arrange those numerals to create 4 digit numbers.
