The answer to this is y=-6
This is because if the line is parallel to the x-axis that means that it is a horizontal line, or in other terms has no slope. This means when you make an equation for the line you don't have a x because it is zero. Now you just need the y-intercept for the equation and because the line is six units below the x-axis that means the y-intercept is -6. Because of all this your equation is simply y equals the y-intercept in this case -6 so you end up with your equation to the line y=-6