that is your formula.
The population mean (mu) is not given, but that isn't important here, because they are interested in the probability that the sample mean is within $500k of population mean. We can just assume the population mean is 0 and put the two sample means at +500k and -500k.
Given the ratio of N to n, safe to say distribution is normal.
Given standard deviation of the population at 2845000 plug everything in.
will be your left z score and
B. (500-0)/(2845000)*(sqrt(26)) will be your right z score
Take Answers from B - Answer from A and should get you your result.