The number of Americans who participate in aerobic exercise in thousands can be modeled by E(a) = −7.5a2 + 523.1a − 3312.3 where E(a) represents the number of people in thousands who participated in aerobic exercise at age a years old. (a) Find the number of 25-year-old Americans who participate in aerobic exercise. (Round your answer to one decimal place.) There are Incorrect: Your answer is incorrect. thousand 25-year-old Americans who participate in aerobic exercise. (b) At what age(s) do 4 million Americans participate in aerobic exercise? (Hint: 4 million = 4000 thousand. Enter your answers from smallest to largest.) About 4 million -year-old Americans and 4 million -year-old Americans participate in aerobic exercise.