Sharyn needs 3.5 yards of fabric to make a dress for herself and 2.25 yards of the same fabric to make a matching dress for her daughter. if she buys 6.5 yards of fabric, how much will she have left over?

You add 3.5 and 2.25 because that is the fabric your needing and you subtract them from 6.5 and get: 6.5 - 5.75 = .75 yards of fabric left over

First you add both of the yards needed for both of the dresses:
3.5 + 2.25 = 5.75 yards

Then subtract the amount of fabric she buys (6.5 yards) by the total amount she needs (5.75 yards) to get how much she will have left over:
6.5 - 5.75 = .75 yards

So Sharyn has 0.75 yards of fabric left.