I need to solve this for my Algebra test review

I like to teach this problem with the guess my rule. If we started out, I'd have you tell me what goes in and I'd tell you what comes out. You'd say 0 and I'd say 2, and you'd say 1 and I'd say 3 and you'd say 2 and I'd say 4 and either you would answer plus 2, keep geussing, or give me a look like the answer is so painfully obvious that my eyes might pop out of their sockets.

And eventually we'd get to this rule, where -2 goes in and 15 comes out, and -1 goes in and 12 comes out. You could tell that x goes up by 1 each time and y goes down by 3, we have a linear relationship and that all we need for that is a starting point (or x-intercept) and a rate of change (y goes down 3 every time x goes up 1) and then you'd think in your head it must by y = -3x + 9 based on the evidence but you'd have to go back and check each answer before you guessed b/c if you didn't and you made a mistake you would lose and I would win and I told you that the loser has to wash the winner's car b/c I knew you were too young to actually have a car.