This is a good problem that has a lot of concepts in it, the most important one being that "negation" you must do between your Cord equations (which are based on variable "s") and your FBD equations (based on variables "x" and "y").
Overall with this problem, you'll get experience with
1) Free Body Diagrams
2) Using Kinematic Equations
3) Writing Pulley Equations
4) Writing Cord Equations
5) Using FBD/Cord "negation" (as described above)
