The 1.645 is the value of z for which the probability of being less than z is 0.95 in a normal distribution, so that the probability of being greater (the tail that is shaded in the graph) is 0.05. It is commonly found from a table. Your textbook may have used this number many times with full explanations, and at this point your teacher may expect you to have practically memorized the number (or at least know where to look it up).