Explanation:
We can rewrite y = -3 as y = 0x-3 or y = 0x+(-3)
Compare that to y = mx+b and we can see that m = 0 is the slope and b = -3 is the y intercept. The slope of 0 indicates a horizontal line.
We can think of it is as slope = rise/run = 0/1 = 0. A rise of 0 tells the reader "the graph does not go up or down at all". Every point on the line y = -3 has y coordinate of -3. Two such points are (0,-3) and (1,-3).
