Hi Elana,
You need to do to things to come up with the equation.
The first is easy: Look where the line crosses the y-axis. That's the y-intercept. See it? It should be -1. In an y=mx+b format, the y-intercept is the "b". Therefore you can eliminate the first and third options since they both show +1 at the end.
The second part is simply finding two points and figuring out the "rise over the run". Let's use where the -1 intercepts the y-axis (0,-1). Now find where there's another good intersection point - let's use (3,0) or the x-intercept. Your rise will be the distance from -1 to 0 (or the difference in "y's"). That's 1. Your run will be the distance from 0 to 3 (different in "x's"). That's 3. So you're rise over run is 1/3. Throw in the "x" afterwards and you have your "mx".
So y= 1/3x-1.
I hope this helps. Should you need any questions, please don't hesitate to reach out. I can provide support through video conferencing too! Good luck!
