I am having trouble understanding what is going wrong when trying to use the Coefficient function on expressions in InputForm. A very simple example serves to illustrate.

In[1]:= a = 3 x + 4 y // InputForm

Out[1]//InputForm=
3*x + 4*y

In[2]:= Coefficient[a, x]

Out[2]= 0

Is there any way to make this work, or convert the old expression back into Standard form? Cheers!