Question 710277
You were already given the answer to the question.  Both dogs still weigh 36 pounds.  


What you could also answer is, how much does each dog weigh.  If you were interested in that question, you could proceed this way:


f for fido, r for rex.
{{{f=2r}}}
{{{f+r=36}}}


A simple substitution for f would be most logical.
{{{(f)+r=36}}}
{{{2r+r=36}}}
3r=36, so r=12.


Rex is 12 pounds, Fido is 24 pounds.