If sin a = 7/25, cos a = 24/25, sin b = 3/5 and cos b= 4/5, find cos (a-b)

Monitors manufactured by tsi electronics have life spans that have a normal distribution with a standard deviation of 1800 hours and a mean life span of 20,000 hours. if a monitor is selected at random, find the probability that the life span of the monitor will be more than 17,659 hours. round your answer to four decimal places.