Plz explain how i'm doing this slope wrong!

Find out the  what was the original price of her purchase .

To prove

Let us assume that the original price of her purchase be x.

As given

A store is having a 20%-off sale on all merchandise. If Mai buys one item and saves $13 .

20% is written in the decimal form.

= 0.20

Than the equation becomes

x × 0.20 = 13

x = $65

Therefore the original price of her purchase is $65.