The total sale price of n shirts can be found using the following expression:
total sale price = n * (26.99 - 5.99)
This expression first calculates the sale price of a single shirt by subtracting the discount from the original price, and then multiplies that value by the number of shirts to get the total sale price.
For example, if Mia bought 3 shirts, the total sale price would be 3 * (26.99 - 5.99) = 3 * 21 = 63.