since the $z$ value is $8.88888...$ after we plug in the mean sample mean, sd and sample size. So we find that this value is greater than $1.645$ which is at the $\alpha = 0.05$ significance level (if we assume this is the point we want to test). Hence we reject the null hypothesis of $H_0 : \mu \le 6.4$. (my statistics is really bad)