Question 417104
lets say X and Y are the "numbers" so it will be

X+Y=15
3x-5y=11

so we know that we can multiply so that you able to cancel out the 3x...so 
we mulitply x+y=15 mulitply it by negative 3 so it can get canceled out. 
so the new equation is -3x-3y=45
so now you can rewrite it as 

-3x-3y=45
 3x-5y=11
   -8y=36
so then 
-8y divided by -8 is just y so then 36/-8 is -4 
so Y=-4