to get to each point you go up 2 to the left 1, up is positive as if u go up in the graph you go y+1 left is negative as to go left you go x-1 so 2/-1 is m(the slope) or -2. now find the y intercept is the point where x and y cross at x=0 which is 4. put it together and you have y=-2x+4
