What are all of the zeros in this equation?

Adepartment store purchases screen-printed t-shirts at a cost of $5 per shirt. they mark up the price 150% (making the selling price 250% of the store's purchase price) and put them on the sales floor. every month that a t-shirt doesn't sell, the store reduces the selling price by 25%.

Find an equation for the linear function which has slope -10 and x-intercept -7

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?