Finance Question for a quick few points?

The cellular spinoff company Jog wants to estimate the proportion of households that would purchase an additional cellular line if it were made available with a free handset. A random sample of 500 accounts is selected. The results indicate that 135 of the accounts would purchase an additional line if the handset was free.

Q-5a: Construct a 99% confidence interval estimate of the population proportion of accounts that would purchase the additional line if the handset were free.