You want to estimate the mean number of sentences in a magazine advertisement. How many magazine advertisements must be included in the sample if you want to be 98% confident that the sample mean is...

You want to estimate the mean number of sentences in a magazine advertisement.  How many magazine advertisements must be included in the sample if you want to be 98% confident that the sample mean is within an error of 3 sentences of the population?  You know the population standard deviation to be σ=7.12