Proportion Testing, which formula o use on TI-84

A researcher wishes to estimate the number of households with 2 computers. How large a sample is needed in order to be 98% confident that the sample proportion will not differ from the true proportion by more than 2%? A previous study indicates that the proportion of households with two computers is 20%.

Where does all of this tie in? I was absent in my class the day they went over this, TIA!