[metapost] solving equatiosn with metapost

[Yogeshwarsing Calleecharan]

Hello,
           I am interested in Metapost's ability to solve equations. 
Consider this code "second.mp":

a/b=(a+3)/(b+8);
a=-3;
show a,b;

When I run the code, however I get mistake(s):

mpost second.mp
This is MetaPost, version 1.504 (kpathsea version 6.0.1)
(mpost.mp (c:/texlive/2011/texmf-dist/metapost/base/plain.mp
Preloading the plain mem file, version 1.004)) (./second.mp
 >> a
 >> b
! Not implemented: (unknown numeric)/(unknown numeric).
<to be read again>
                    =
l.1 a/b=
         (a+3)/(b+8);
?


I would be grateful if someone would let me know:

(a) what I am doing wrong with this code?
(b) Say if the code is fixed and run well, what Metapost command to put 
after show a,b; so that Metapost exits normally because it seems that I 
get a * if the code runs well and then Metapost is still awaiting for 
some other command(s)?



Thanks a lot...

Best Regards,
Y.Calleecharan