How can I make the r.h.s solely dependent on x(i.e. a function of x)? We have 
$$ a = \exp(-x/b) x$$

I tried taking the log on both sides, but I face a similar wall in the simiplification, namely:
$$ln(a)=ln(x)-x/b$$ still cannot get rid of b on the r.h.s.
Any suggestions would be much appreciated.