Normal distributions are symmetric, so if the mean is 56 and Eighty percent of the values are greater than 48, then eighty percent are less than 64.

Do you know how to find a Z-score for 80%? You can use a table(like the one on the tab at the top of this page) or a Z-score calculator

Once you have the Z-score

Z-score = \(\frac{x-\mu}{\sigma} = \frac{64 - 56}{\sigma}\)

Solve for \(\sigma\)