The monthly sales of mufflers follow the normal distribution with a mean of 1350 and a standard deviation of 250. The manufacturer would like to establish inventory levels such that there is only a 6% chance of running out of stock

Ok so I know that getting the answer is by using the formula x=zs+u BUT how do I figure out what my Z is?? Where do I look for it? And in which appendix or z curve tables do I look for it in?? PLEASE HELP. Thank you.