How many 3-digit numbers can be formed using the digits 2 3 5 6 7 9 if repetition of digits is not allowed?

This is a basic permutation problem. You have 6 elements, of which you need to select 3, without replacement, and order does matter (123 is different from 321).

Permutations are covered by the wikipedia page on permutations. In short, however the answer is P(6, 3) = 6!/(6-3)! = 6*5*4 = 120.