When figuring out the equation you need to find the change in y and the change in x. First, figure out how much more or less the first two values change. It goes from 1 to 2. Which means its adding 1. In y you noticed it is SUBTRACTING by 1 in the first two values. So, once you get those 2 values you divide the change in y by the change of X( 1/-1). The reason it would just be -x is because x means one. If you need more clarification, let me know!
