The width of a rectangle is three more than twice its length

Triangle xyz has vertices x(1,3), y(0,0)

Ateacher wanted to buy a chair, a bookshelf, two tables and a desk. she spent $900 for all five items and the chair and the desk combined 70% of her total. if the bookshelf cost $50, how much did each of the tables cost?

What are the triple angle formulas? is it related to double angle identities?