If the letters can be repeated,

Then, number of combinations = 6*6*6 =216 (as there are 6 letters)

 If the letters can't be repeated,

Then, number of combinations = 6*5*4 =120 (as the letter which has been used cant be used again)

 case 1: If both A's and a different letter are selected

case 2: If all the selected letters are different

Total no: of arrangements= 72