Explanation
She saved 856.99 - 728.44 = 128.55 dollars.
Divide this over the original price.
128.55/856.99 = 0.150002 approximately
Move the decimal over 2 spots to the right to get 15.0002% which is also approximate. It's probably best to say round to the nearest hundredth. Doing so leads to a 15% discount approximately.
If your teacher tells you to round another way, then be sure to follow those instructions.