How do I "write up an R.O.I."?

I've spoke with the manager of my local Applebees restaurant about running a weekly pub trivia game, he said he's willing to hire me at $75 a week with a Return On Investment (ROI) of 4 times that. That much I understand, but then he said to "write up an R.O.I." he could submit to his supervisor... That would just be a number (you'll make an extra $350 a night if you hire me at $75 a night) ...what else do I put on the paper?

He wants to know how you intend to earn him that $350. How many extra people do you plan on pulling in, what will they spend, what is the profit margin (some of this data you may need from Applebees).