Answer :

if y=2x 
 7y=14x .

6x+14x=20
20x=20
x being 1 x=1

we found x .
and putting in this problem.

y=2x
y will be 2 , y=2

i am so sorry for my bad english :(
konrad509
[tex]6x+7y=20\\ y=2x\\\\ 6x+7\cdot2x=20\\ 6x+14x=20\\ 20x=20\\\\ x=1\\\\ y=2\cdot1\\ y=2\\\\ \boxed{(x,y)=(1,2)} [/tex]