Really, there are an infinite number of right answers, but you're probably looking for a linear equation. In which case, we want an equation of the form y = mx + b. m is the slope and b is the y-intercept.
(For this problem, if you just think about it for a minute, it's pretty easy to see that it'll be y = -x, but I can go through the process of how to do it for more complex problems, if you like.)