What are the allowed values of a in ln(2x+y)=a

The curved surface is the plot of ln(2x+y). The planes are z=1, z=0 and z=-1.

So we get ln(2x+y)=1, or ln(2x+y)=0 or ln(2x+y)=-1. In other words, the right side of

ln(2x+y)=a can be any number.