What is the LCCDE equation of this block diagram?
I assume D is a "sample" delay making (e.g.) e[n] into e[n-1]. Use the standard form for a delay of the multiplier \$Z^{-1}\$ and just treat it like algebra i.e. 

e[n-1] = e[n]\$\cdot Z^{-1}\$

The rest should be easy.