Let x be the original price of the boots (before the discount is applied)
If the discount is 15% and you saved $57, then this means
15% of (original price) = amount saved
15% of x = 57
0.15x = 57
x = 57/0.15
x = 380
So the original price is $380. This is the price before the 15% discount is applied.
After you apply the 15% discount, the price lowers to 380-57 = 323
So the final discount price is $323