Question 37049
I think that you mean:
x+2y=5
3x-15=6y
If that is the system I will show you how to solve it.

By Elimination


(1)x+2y=5   We place the x and the y where they must be
   3x-6y=15  

(2) We multiply the first equation by 3 in both sides to obtain a similar equation.
   3(x+2y)=3(5)
    3x-6y=15  

(3) 3x+6y=15   Now we obtain this system.
    3x-6y=15 

(4) Finally we sum both equation obtaining a new one wich only one variable.

    3x+6y=15   We obtain 6x=30, then x=5, and therefore y=0
   +3x-6y=15 


By Substitution

   x+2y=5  We leave one variable alone, x=5-2y
   3x-6y=15  We substitute the variable in the seconde equation 

   3(5-2y)-6y=15
   15-6y-6y=15
    -12y=0 and therefore y=0

   Now from the first equation
  
  x+2(0)=5
  x+0=5 And therefore x=5


Here are the solutions, I hope that they are useful for you.