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.

3(15-y)=5y-11

45-3y=5y-11

8y=56

y=7..

And then..

x=15-7