No, answer is not 6;7. How could 6+7 be 15?
Well.. you got one equation right ( 3x=5y-11 )
Since you have two variables you also need two equations. And the other one would be in the first sentence I wrote. (the sum of those two variables is 15).
So it is x+y=15.. Then you could write out X, which is x=15-y and then you could just replace it to the other equation. Rest should be easy enough.