I take it b is the length of the smaller rectangle.
The first thing I would do is to label things. Starting at the top right and moving clockwise, label the corner points of the large rectangle A, B, C, and D. Starting with the highest and moving clockwise, label the corner points of the small rectangle E, F, G, and H.
Now let x = length of line segment DG. Let y = length of line segment DH. What is the length of GH?
What is the length of CG expressed algebraically? What is the length CF expressed algebraically?
Now can you prove a relationship between triangle DGH and triangle CGF?
