You need to analyze the information given in the exercise:
1. The regular price of the pair of glasses was $72.
2. It was 30% off when she bought the pair of glasses.
By definition, a percent can be converted to a Decimal number by dividing it by 100. Therefore, you can express 30% as following:

Let be "x" the amount of money (in dollars) that Gabriela paid for the glasses with the discount.
Using the information given in the exercise, you can set up the following equation:

Finally, evaluating, you get this result:

The answer is: $50.4