One costume uses 6 yards of material, and she has used 48 yards. So by dividing 48 by 6 you can identify the number of costumes she has made.
One costume needs 3 yards of trim, so take the number of costumes she has made that you calculated above, and multiply by 3.
48 ÷ 6 = 8
8 x 3 = 24
Therefore she will use 24 yards of trim