extracting 'pieces' of vector element



Suppose I have the element of some structure (say a vector), which is an 
expression.

e.g.,  [N=26.5]

I want to 'grab' the rhs of the expression (i.e., the number 26.5), but 
can't figure out how. Suppose the preceding is assigned to Y

I thought the following would work

rhs(Y);

but it keeps returning 0.

I'm obviously missing something.

Thanks in advance for pointers to the obvious.