I need to know how to do this im really confused and i dont know how to solve it

What is the value of y? round to the nearest tenth

Bring the fraction a/a-4 to a denominator of 16-a^2

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?