Cereal boxes labeled 500 g, but the fill looks light

A cereal brand labels each box as 500 g. The filling line's standard deviation is known to be $\sigma = 12$ g. A regulator weighs 64 boxes and finds a mean of 497 g.

Test at the 5% level (two-sided) whether the true mean fill differs from 500 g, and report the z-statistic.