Hey! Let's go through this.
If she has 42 dollars and she buys 3 $7 t-shirts, we can use multiplication to see that she spent 21 dollars on t shirts, meaning she has 42-21 dollars left, or 21 dollars left.
If she has 21 dollars left and glasses sell for $6, then the most she can buy (keep in mind we can't buy a fractional part of a pair of sunglasses) at most 3 pairs, because if she bought four, she'd go over her spending limit.
Hope this helps!
