HELP URGENT!!! There is an answer but it is incorrect because I forgot to tell him to use the substitution method.

y = 2x 

y = 3x + 1

I'm not certain what you're asking.  Solve for x?  Just set the two expressions equal.

                                                 3x + 1 = 2x

Subtract 2x from each side       3x – 2x + 1 = 2x – 2x

                                                   x + 1 = 0

Subtract 1 from each side           x + 1 – 1 = 0 – 1

                                                    x = –1

x = –1

The guest has used the substitution method.....

And since  x  = -1

Then y  =  2(-1)  =  - 2