Looking at just the plots, you can see that they are equal in y-value, so they would make a horizontal line, meaning no slope. That just leaves the y-intercept, and if it is horizontal, and they are both at y level -3, the intercept is -3. So putting it in slope-intercept form, you get y = 0x - 3. The 0 signifies the line doesn't slope down or up, just horizontally.
