Explanation:
what does it mean for an object to stay at the same height ?
come on, you cannot tell me you don't understand this.
it means that e.g. the object is at a certain height (e.g. 50 ft above the ground) at the beginning of a time interval. then 1 seconds later it is still at the same height. 1 seconds after that it is still at this height. and so on.
how do you think that shows on a chart that displays height on an up or down scale ? as a flat (horizontal) line.
the chart is simply a rough picture of what the object does in real life.
so, where do we have flat (horizontal) segments in the curve ?
between second 2 and second 6.
and then infinitely after second 12.
in interval notation
[2, 6] and [12, +infinity)
please note the round bracket on the infinity side. it means the indicated end of the interval is not included. that is, because infinity is just a concept and not a number. it can never be reached. and it is therefore not included in the interval itself.
FYI - the flat line between second 2 and second 6 is not possible in real life, unless it rolls on a flat surface or something is holding it steady in place for these 4 seconds.
but a flying ball can never have a pure flat curve regarding height. only when it comes at rest on the ground. gravity would not allow that. not even for a bullet shot by a gun.