The letters portion is easy; there are 26 choices.
The sequence of three digits containing at least one 1 and at least one 2 is a little trickier. One way to approach it is to break the possibilities into three cases:
1. Arrangements of 1, 2, x where x is a digit other than 1 or 2
2. Arrangements of 1, 1, 2
3. Arrangements of 1, 2, 2
Add up the number of possibilities in the three cases, multiply by the 26 possibilities for the letters, and that should do it.
