Notice that the line on this graph is a flat line or a horizontal line.
When you are asked to write the equation of a horizontal line,
your equation will always be in the form of y = a number.
When y = a number, the graph is a horizontal line or a
flat line whose y-intercept is equal to that number.
Since the y-intercept is the point where the line crosses the y-axis and our line crosses it at the point (0, -1), the equation of this line is y = -1.