You pick a 2-character code. First character: letter (26) or digit (10). Second character: any letter (26).
First position: letter OR digit, so 26 + 10 = 36 choices. Second position: 26 choices. Both positions required: 36 × 26 = 936 codes.
Notice you used both rules. Sum rule for first position (OR), product rule to combine positions (AND).