Please help I will give Brainliest. The pie store is having a 20% off sale on all of the pies if the pie regularly cost 18$ how much would you save with the discount.

Sanderson is having trouble with his assignment. his shown work is as follows: − 3 7 7 3 = − 3 7 × 7 3 = − 21 21 = −1 however, his answer does not match the answer that his teacher gives him. complete the description of sanderson's mistake. find the correct answer.