This is a 'multiplication principle' problem. If you have 3 events in order, say E1 E2 E3... And E1 has N1 ways of doing it, E2 has N2 ways of doing it and E3 has N3 ways of doing it, then there are N1 * N2 * N3 ways of doing E1 then E2 then E3.
So, look that up in your textbook. In this case there are 26 * 25 possible ways to pick a 2 letter sequence without repeating letters. Hope that is helpful.