So since you're supposed to use substitution and both equations are for Y, you can plug either in and it would give you the same results.
2x = x + 6
-x -x
x = 6
Then to find Y, plug your new X value into one of the equations.
Y = 2(6)
Y = 12
Check your work by plugging your answers in.
12 = 2(6)
12 = 12
