Finding an expression for the transfer function of a discrete system

I want to ask you how it is possible to find an expression for the transfer function of a discrete system!

It is a simple model but i do not know how to find the tranfer function of the system.

You can imagine a simple simulink model with the components: -Signal input -Gain A -Gain B -Sum -Unit Delay (discrete 1/z) -Signal output

Have you any idea how can i derive the trasnfer function of the system?

Thank you in advance!