I am confused on this problem as I do not know where they get the 0.9974 from when they take the probability of p(-3

Problem 7 In producing ball bearings, the manufacturing process has shown a radius variation of o' =0.09mm-. Using Chebyshev's inequality, it can be shown that the upper bound on the probability that the radius falls more than 0.9 mm away from the mean is 0.11. Suppose now that the radius is normally distributed. Determine the actual probability that the radius will fall 0.9 mm away from the mean and compare your result to the upper bound