How to find midpoint of a line segment

What is the image of c for a 90° counterclockwise rotation about a? (3, 7) (-1, 3) (7, 3)

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?

Find the length of the normal and the angle it makes with the positive x-axis. (see pictured below)