Given:
Melissa paid $12.24 for a 6.35-kg.
In another store, she paid $13.99 for a 7.48-kg.
To find the unit price:
The unit price for a first store is,

The unit price of the second store is,

So, the answer is,
The unit price for a 6.35-kg is $1.93 per kg.
The unit price for a 7.48-kg is $1.87 per kg.
So, the better buy is the 7.48-kg bag.