Answer:
If your slope is Zero, then you know that your line will be a horizontal line crossing the y-axis at a specific value. If one of the of the points on that line is is (4,7) then you can draw a horizontal line through that point. Notice that when you do that the line only crosses the y-axis at the point y=7. Hence your equation y=7
Helpful Hint
Slope = 0, Horizontal line, equation is y = a #
Slope = Undefined, vertical line, equation is x = a #
Explanation: