15 less than a number S is three more than 9 times a number T. Solve for S

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%.

Me complete this proof! prove that a quadrilateral is a square. me with the steps for this proof.

Find the missing dimension extra points