The probability of any particular event occurring can be expressed as:
(number of desired outcomes)/(total number of outcomes)
Here, the total number of ways you can pick a single letter from the word "mathematics" is 11 (m, a, t, h, e, m, a, t, i, c, s), and the number of a's available to choose from is 2, therefore you have a 2/11 chance of choosing an "a" at random from the word "mathematics."