For this case we must find the number of miles that Shayna ran in a month.
We know he ran 6 miles in a week.
So, let "x" be the number of miles that Shayna ran in a week:

Let "y" be the number of miles that Shayna ran in a month, so if what she ran in a week was
of what she ran in a month we have:

So, Shayna ran 18 miles in a month
Answer:
18 miles