Equations of motion - equilibrium condition 
Two frictionlessly shiftable mass points are connected by a massless thread of constant length l. For the arrangement given by the figure, use D'Alembert's principle to determine the equations of motion and the equilibrium condition.


I'm learning for exams and always struggle with D'Alembert's principle. I asked my teacher for some exercises to practice it and this is one of them. Apparently it's one of the easier ones but I'm struggling with it.
I've read the wiki entry again and am no smarter than before. And it kind of seems that this all depends on $\alpha$ and $\beta$, but since they are not explicitly defined I don't know how to approach this. Can someone explain D'Alembert's principle in simple words or give me some hints on how to solve this type of questions? Can't seem to grasp the concept.

This is one of the most common questions you will see in physics textbook of asian high schools( i dont know about curriculum of US and other western nations) this type of question falls into category of "Newton's laws of motion and its applications" if you are not familiar with laws of motion i suggest you to read the the book "Statics and dynamics by S.L. loney"(it's a classic!)
Anyway here is my approach to solution:
1)Since masses move frictionlessly over the surface so it means that friction(f)=0 on the surface of inclined plane or we can say surface is smooth
2) now the whole system (inclined plane and two masses) moves with acceleration 'g' so this is moving or dynamic frame 
This link will provide you some knowledge about d alembert principle and using it to solve this specific problem

