Well, your diagram is open to some interpretation. Is it a static figure? Is there friction present for the surface on which M rests? Are D and M supposed to have the same mass?Why is t1=t2? Has it something to do with newtons third law, u guys dont explain very well sorry..
Well, the easy answer is it's the same rope. When you pull on one end with a certain amount of force, that same force is transmitted all the way back to the point where the other end of the rope is attached. That's the First Law of Ropes... This dosent help me, just think of a normal pulley problem where T1 is equal to T2, but why? They are equal, but is it because its "one string" or because of "newtons third law" or why? like common, this is a very easy question, but its quite hard if ur teacher havent told you (in this case told me)