Force between two steel blocks connected by a string

[drinkingstraw]

Two steel blocks are at rest on a frictionless horizontal surface. Block X has a mas of 3.0 kg ans is attacthed by means of a light taut string to block Y that has a mass of 12 kg. A force of 45 N (E) is applied to block x. Calculate the force block Y exerts on block x.

I don't have any idea of how to go about do this question. I think it's related to Newton's 3rd law about action-reaction forces but I don't know any else. Could someone please guide me in the right direction? Thanks

 

[Astronuc]

There is a taut string - in tension.

The force is applied to one block (X), but because of the string, it affects both blocks of the total mass, and X is pulling on block Y by virtue of the string.

This might help - http://hyperphysics.phy-astr.gsu.edu/hbase/fcm.html#c1

 

[drinkingstraw]

There is a taut string - in tension.

The force is applied to one block (X), but because of the string, it affects both blocks of the total mass, and X is pulling on block Y by virtue of the string.

This might help - http://hyperphysics.phy-astr.gsu.edu/hbase/fcm.html#c1

Thanks :D