A consumer electronics company is comparing the brightness of two different types of led screens for use in its television sets. Screen type a has mean brightness of 75 and standard deviation of 8, while screen type b has mean brightness of 70 and standard deviation of 12. A random sample size 36 is selected from screen type a where the sample size for screen type b is 25. Find the approximate probability that x¯ a − x¯ b exceeds 4?