Mth Math Math Math Plz

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%.

Write an equation for the function that includes the points (2,100)and (3,100)

Having a hard time understanding this question (see attachment).