Answer
Number of different ways call letters can be formed = 2$\times$26$\times$26$\times$26 = 35,152
Work Step by Step
The first letter must be W or K, then the number of ways to express first digit = 2
The number of way to express second letter = 26
The number of way to express third letter = 26
The number of way to express fourth letter = 26
Number of different ways call letters can be formed = 2$\times$26$\times$26$\times$26 = 35152
