She bought 'x' number of shirts.
The price of one shirt = 12.25 dollars.
Then total price of all shirts would be '12.25x' dollars.
Given is the shipping charge = 3.99 dollars.
Given is the total bill = 77.49 dollars.
It says no other tax would be charged on this order, so total bill would be sum of total price of all shirts and shipping charges.
Total Bill = Total price of all shirts + Shipping charges.
77.49 = 12.25x + 3.99
Hence, option D is correct i.e. 12.25x + 3.99 = 77.49
