If she is making 6 costumes that need 3 7/9 yards each, the equation would be 6 x 3 7/9.
The answer to that equation would be 22 2/3.
How to get the answer:
3 x 6 = 18
7 x 6 = 42 (the seven is from the 7/9)
42/9 = 4 2/3
then add the eight teen and the 4 2/3 and you get 22 2/3.