If a linear graph of $\displaystyle \frac{do}{di}$ as the y axis variable vs "do" as the x axis variable is graphed, what would the slope and y-intercept be?

I think I've reached the conclusion that the slope will become $\displaystyle \frac{1}{f} $ but I'm not sure how to figure out what the y-intercept would be, I mean if you set the x-axis, or "do" to 0 wouldn't the y-axis of do/di also become 0? (the only given options were 1 or -1)