Ahmad is choosing a 2 letter password from letters a, b, c, d, e, f. the password cannot have the. same letter repeated in it. how many possible combinations

Since order matters (but no repetition= one letter can be used ONLY ONCE)
then it's a permutation :

⁶P2 = 6!/(6-2)! = 30
Another method:
There are 6 ways to chose the 1st letter, and 5 ways to choose the 2nd (because one letter already chosen), the there are 6x5 = 30 ways

He will need 2,898 pieces of paper