Total quantity of milk bought by Mrs Quirk = 1 quart
Quantity of milk Michael drank from it = 2/7 quart
Amount of milk drank by Joe from it = 1/3 quart.
So the total amount of milk drank by Michael and Joe = (2/7) + (1/3)
= (6 + 7)/21
= 13/21 quart
Then
Amount of milk left = 1 - (13/21) quart
= (21 - 13)/21
= 8/21 quart
So 8/21 quart of milk is left after Michael and Joe drank from the 1 quart bought by Mrs Quirk. I hope this is the answer you were looking for and the procedure of doing this problem is clear to you.