there are 2 pints in 1 quart
so if you change the quarts to pints and then subtract then you can change your answer back to quarts and pints.
6 qt and 1pt = 13 pints
2 qt and 1 pt = 5 pints
13 - 5 = 8 pints
your answer should be 4 quarts
hope this helps :)