The alphabet is among the first things people learn when they study the English language. Remarkably, quite a number of advanced speakers have problems pronouncing the letters. In particular, the last three letters, x, y, and z cause most of the problems.
X is correctly pronounced as [eks], listen here. A common mistake is the use of the German pronunciation, [iks]. It is not exactly helpful that in German, English names such as X-Drive or X-Men are often pronounced by mixing the German pronunciation of x with the English pronunciation of drive and men. Two science-related examples are x-rays and the XXZ model.
Z is pronounced as [zed] in British English, but as [ziː] in American English, listen here. Very often, people mix a British accent with an American z, or the other way around. Also, note that the American pronunciation of z is not the same as that of the letter c [siː], listen here; the difference can be quite important when you have c and z in an equation you are talking about.