Answer:
The domain represents the x-axis, more specifically, what is happening on the x-axis. So when looking at a graph, if you are asked to find the domain think about what the x-axis looks like. I put an image in to show you an example of what the domain would be for a parabola:
So on the left side of the x-axis, we can see that the line stretches out into negative infinity, so the domain would begin at negative infinity.
On the right side of the x-axis, the parabola also stretches into positive infinity, so here the domain would be (negative infinity, positive infinity), because it goes to both ends forever.
