Question 759439
You did well and then you found NO obstacle other than believing you found an obstacle.  Those two resulting equations are correct.  TWO equations and use TWO variables, x and y.  You have this system now, simplified from what you made initially (which was obviously also correct):


SYSTEM:
{{{x+y=3}}}
and
{{{2x-y=18}}}


All you need to do is just FINISH!  Two equations, both linear, in two variables.
You have no reason to feel stuck.  
... Try adding one equation to the other!  That would eliminate y and you would then solve to find a value for x; then go back and find the value for y.


{{{x+y+(2x-y)=3+18}}}
?