A random sample of size n1 = 16 is selected from a normal population with a mean

Question:

A random sample of size n1 = 16 is selected from a normal population with a mean of 75 and a standard deviation of 8. A second random sample of size n2 = 9 is taken from another normal population with mean 70 and standard deviation 12. Let and be the two sample means. Find
(a) The probability that exceeds 4
(b) The probability that