The first letter can be any one of 26 . For each of those . . . The second letter can be any one of 26 . For each of those . . . The third letter can be any one of 26 . For each of those . . . The first digit can be any one of 10. For each of those . . . The second digit can be any one of 10. For each of those . . . The third digit can be any one of 10. For each of those . . . The fourth digit can be any one of 10.
You can multiply together the number of possibilities for each part of the password. Since there are 26 letters and 10 digits to choose from, the calculation is 26 • 26 • 26 • 10 • 10 • 10 • 10.