Remember that z has a standard normal distribution. This solution is as simple as finding that number in a table in the back of your book. This is the same as asking what is the probability that z is less than 2.55 standard deviations above the mean. Recall that the probability that z is less than 1.645 is 0.95, so we know that the probability that z is less than a value greater than that will be greater.

~Matt