24: The derivative

is (when taken in absolute value) a decreasing function on the relevant range, so its maximum is attained at the left endpoint. Thus

The x in the denominator is rather unpleasant. We can get a more reasonable formula like this. We want approximation for small x anyway, so it is no problem to restrict our attention to |x| < a²/2. Then we can estimate in the denominator like this:

Answer