assume all date is bell shaped please show all work for this problem

4 a) A tire company manufactures tires with a mean life of 43,000 miles and a standard deviation of 3900 miles. The company wants to issue a guarantee for the tires, but does not want to replace more than 4 % of the tires sold. How many miles should the tires be guaranteed for? (Hint, watch out for the sign of Z.) b) If the company expects to sell 90,000 tires in a year, how many tires can the company expect to replace under the guarantee