1. **State the problem:** Find the equation of a graph that is a horizontal line with a y-intercept of -3. 2. **Recall the formula for a horizontal line:** A horizontal line has the form $$y = c$$ where $$c$$ is a constant. 3. **Use the given information:** The y-intercept is the point where the line crosses the y-axis, which is at $$y = -3$$. 4. **Write the equation:** Since the line is horizontal and crosses the y-axis at -3, the equation is: $$y = -3$$ 5. **Explanation:** A horizontal line has a slope $$m = 0$$, so the equation can also be written as $$y = 0 \cdot x - 3$$, which simplifies to $$y = -3$$. This means for any value of $$x$$, $$y$$ remains constant at -3. **Final answer:** $$y = -3$$