2x+3y=15
x+y=6
what are the values of x and y

They're are 3, if you substitute in 3 for both variables it equals 6 and once doing that you can plug it into the equation above it and do PEMDAS, hope I helped :)

Actually you would use elimination. You would multiply the second equation by -2 so you could eliminate the x variable in both equations
    2x+3y=15
    -2(x+y=6)
   
so now you have...
2x+3y=15
-2x-2y=-12

the x variables cancel out, and now you are left with
3y=15
-2y=-12

you combine these two equations now using addition, then solve like a normal one-step equation
3y-2y= 1y
15-12=3

now your equation is 1y=3 or y=3

hope that made sense :)