We know that Logan bought 8 t shirts and that each of them cost $6.50, this means the he spent:

on t shirts.
Now, we know that he bought 4 pairs of shorts and that each of them cost d dollars, this means that he spent:

on shorts.
Finally we know that the total was $84, so addind the first two expressions and equating to the total cost leads to:

Solving for d we have:

Therefore, one pair of shorts cost $8.00.