\(\displaystyle |x^2- 64|< 0.001\) is the same |(x- 8)(x+ 8)|< 0.001. If x is close to 8 then x+ 8 will be positive so we can write that as (x+ 8)|x- 8|< 0.001 and then |x- 8|< 0.001/(x+ 8). The largest value of a fraction, with fixed numerator, occurs when the denominator is smallest.