You have the following infromation:
- each t-shirt costs $6
- each shorts costs $9
- She spent $75
- She bought 1 pair of shorts
Then, if Navy bought only 1 pair of shorts, she spent two times the money of one shorts, that is, she spent 2 x $9 = $18. She spent $18 by the pair of shorts.
This means that after buying these shorts she had
$75 - $18 = $57
Then, Navy had $57 to buy t-shirts.
If you divide the rest of Navy's money between the cost of each t-shirt, you can obtain the number of bought t-shirts, just as follow:
57/9 = 6.33
This means that Navy was able to buy 6 t-shirt (the result was a decimal, however, you consider only the interger part)